My computer is on the fritz.

Reinstalling OS X Without Erasing the Drive

Boot to the Recovery HD: Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Reinstalling OS X Without Erasing the Drive

Repair the Hard Drive and Permissions: Upon startup select Disk Utility from the main menu. Repair the Hard Drive and Permissions as follows.

When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list. Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu.

Reinstall OS X: Select Reinstall OS X and click on the Continue button.

Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.

Alternatively, see:

Reinstall OS X Without Erasing the Drive

Choose the version you have installed now:

OS X Yosemite- Reinstall OS X

OS X Mavericks- Reinstall OS X

OS X Mountain Lion- Reinstall OS X

OS X Lion- Reinstall Mac OS X

Note: You will need an active Internet connection. I suggest using Ethernet

if possible because it is three times faster than wireless.

You probably ran a defective third-party "utility" such as "CleanMyMac" or "MacCleanse" that purports to "clean up" "junk" files. What it really does is to corrupt the operating system and many applications.

The first thing you need to do is remove that software according to the developer's instructions. Never install it, or anything like it, again. Then see below. If you use any iWork applications, you may need to delete them and redownload them from the App Store.

If you don't already have a current backup, back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.

There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode. All other data will be preserved.

*The linked support article refers to OS X 10.10 ("Yosemite"), but the procedure is the same for OS X 10.7 ("Lion") and later.

You're not following the instructions linked to my comment. You don't need to open the App Store. You need to start up in Recovery mode.

Did you follow the instructions I provided? I don't believe you did. Or you did not read the instructions.

looks like i need to find someone with a mac, get them to download yosemite

No, that would be piracy. The easiest solution to your problem would be to make a “Genius Bar" appointment at an Apple Store. They can install the OS for you at no charge and also check the machine for a missing serial number or a hardware fault.

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