Q: My mac pro wont open up. I press power on and it begins loading woth bar filling up and when is complete. it turns off again. Help!

Could you be more specific? What kind of Mac Pro? Can you post a screenshot?

begins loading the bar

It only does that when the Disk Drive will not mount. It is running the equivalent of Disk Utility (Repair Disk).

--If it passes, it will boot up when complete.

--If it fails, there is no point staying powered up when starting up is impossible, so it shuts down.

You need to boot to an alternate source of Mac OS X software or Recovery_HD and attempt the (Repair Disk) from there. If you get errors, sometimes the exact error message can be helpful.

¿Do you have Backups?

So you do not have Lion saved on a USB stick? What happens when you hold down command and R at startup?

Ok so Internet recovery works so you can use that to reinstall your OS when you're ready for that step.

Are you sure up you already erased your drive? If you don't have any backups, you'll either need to use online utilities or a service to recover your files.

The online recovery gives my an error message each time.

...and what does the error message say?

Computers that ran Tiger do not run Mountain Lion.

Leopard is the oldest that an eligible Mac Pro would have shipped with in the Early 2008 3,1 with 10.5.1+

Internet Recovery does require a disk drive and free space to setup and hold the download and to initiate an installation. Haven't done so so I don't have first hand as to all the t hings that can and do go wrong.

When you do have the installer package, and BEFORE you run it, SAVE IT SOMEWHERE SAFE!

http://arstechnica.com/apple/2013/10/how-to-make-your-own-bootable-os-x-10-9-mav ericks-usb-install-drive/

When Apple released OS X 10.7 two years ago, it stopped selling operating system DVDs in its stores, stopped shipping recovery disks with new Macs, and switched to downloadable installers for OS X upgrades. These download-only installers have actually worked pretty well—I’ve never had an issue downloading the software from the Mac App Store or restoring a Mac using the Internet Recovery feature when something went south. That said, it’s still nice to have an install disk handy for those cases when you don’t have a connection, when your connection is slow, or when you just have a whole bunch of Macs and don’t want to have to download the installer on each and every one of them.

The good news is, as with Lion and Mountain Lion, it’s possible to create a local USB installer for Mavericks. The bad news is that it wasn’t as simple as it was before—Apple has changed the way the installer works, and making an install disk manually is more difficult than it used to be. Before we get started, here’s what you’ll need:

• An 8GB or larger USB flash drive, or an 8GB or larger partition on some other kind of external drive

The OS X 10.9 Mavericks installer from the Mac App Store in your Applications folder. The installer will delete itself when you install the operating system, but it can be re-downloaded if necessary.

The latest beta of the Lion Diskmaker app, available here (as of this writing, version 3 beta 3 is the most recent). This app is free to download, but the creator accepts donations if you want to support his efforts.

An administrator account on the Mac you're using to create the disk.

https://discussions.apple.com/docs/DOC-3353

Install or Reinstall Mavericks or Mountain Lion from Scratch

the following procedure will remove everything from the hard drive.

OS X Mavericks- Erase and reinstall OS X

OS X Mountain Lion- Erase and reinstall OS X

OS X Lion- Erase and reinstall Mac OS X

About TimeMachine backups and Recovery Mode

http://support.apple.com/kb/HT1427

http://www.apple.com/osx/recovery/

OS X Lion and later let you:

• Start from OS X Recovery using your Time Machine backup disk. With OS X v10.7.3 and later, Time Machine backs up the Recovery System on your Mac to your backup drive as well. This lets you use your Time Machine drive to start up your Mac if needed.

OS X Recovery:

http://support.apple.com/kb/ht4718

OS X Internet Recovery

Mac models introduced after public availability of OS X Lion include the ability to start up directly from an Internet-based version of the OS X Recovery system. OS X automatically uses this feature when the Recovery System on the hard disk isn't available (such as when your hard disk encounters an issue, or when your hard disk has been replaced or erased). OS X Internet Recovery lets you start your Mac directly from Apple's servers. Starting up from this system performs a quick test of your memory and hard drive to check for hardware issues.

OS X Internet Recovery presents a limited interface at first, with only the ability to select your preferred Wi-Fi network and, if needed, entering a passphrase. Next, OS X Internet Recovery downloads and starts from a Recovery System image. From there, you are offered the same utilities and options as a local Recovery System.

...

Requirements for reinstalling OS X using Recovery

Reinstalling OS X using OS X Recovery requires broadband access to the Internet via Wi-Fi or an Ethernet connection. OS X is downloaded over the Internet from Apple when OS X Recovery is used for reinstallation

If you use Internet Recovery to reinstall OS X, it installs the version of OS X that originally came with your computer. After installation is finished, use the Mac App Store to install related updates or later versions of OS X that you have previously purchased.

What to do if the installer warns that no Recovery System can be created

Some disk partition configurations may result in the OS X installer reporting that it could not create a Recovery System. In these situations, even if you are permitted to continue the install, you may want to quit the installation and create an external, bootable OS X hard drive with a Recovery System, first. You can continue your OS X upgrade on your computer's startup drive after creating an external Recovery System.

• Your storage device must have at least 13 GB available (after formatting) to install OS X Lion or later and a Recovery partition.
• These steps erase and reformat the storage device. This article instructs you on setting up a storage device to use the GUID partition scheme and the Mac OS Extended (Journaled) format, which are required to install OS X and a Recovery partition on your external storage device. You should back up any important files that are on the device to a different drive.
• This procedure installs a version of OS X that is compatible with the Mac it was created with. Using this OS X system with a different Mac model may produce unpredictable results.
• Your computer's serial number is sent to Apple as part of this process to help authenticate your request to download and install OS X Lion.

http://support.apple.com/kb/ht4718

Installing OS X on an external volume - such as your USB flash drive

http://support.apple.com/kb/HT5911

So use command+R for internet recovery

Once there, use disk utility to erase your HD back to a single partition

Exit out of DU and install OSX onto your drive