Start up crashing.

Frankiews98,

if you boot into Safe mode, does it behave in the same way?

Frankiews98,

which model MacBook Pro do you have, and which version of OS X is installed on it?

Frankiews98,

you can try this: boot your MacBook Pro into Recovery mode by holding down a Command key and the R key as it starts up. Once the OS X Utilities menu appears, select Disk Utility. On the left-hand side of the Disk Utility window, select your internal disk’s boot partition (typically called “Macintosh HD”). On the right-hand side, press the Verify Disk button if it’s not greyed out; if it is greyed out, or if it reports that errors were found, press the Repair Disk button. Once the verification/repair is completed, exit Disk Utility and select Restart from the Apple menu to restart in normal mode. Are you able to get to the login screen now?

Frankiews98,

if your disk is unrepairable, then you can either:

• back up what you can from it, wipe the disk, and do a clean reïnstallation of OS X onto it; or
• purchase a new disk, format it in Disk Utility, install OS X on it, and boot from it using Startup Manager.

Frankiews98,

boot into Recovery mode and run Disk Utility there. Select your internal drive from the list on the left-hand side of the window. On the right-hand side of the window, select the Erase tab, then select “Mac OS Extended (Journaled)” from the Format dropdown, provide a volume name in the Name textbox (“Macintosh HD” is typical), and press the Erase… button. When the erasure has finished, exit Disk Utility and select Shut Down from the Apple menu.

If you have an Early 2011 MacBook Pro and it originally came with Snow Leopard installed (and two grey installation DVDs in the box), then you’ll need to boot from its grey Mac OS X Install DVD to put Snow Leopard back on the erased disk. If you have a Late 2011 model or an Early 2011 model that originally came with Lion installed (and no grey installation DVDs in the box), then you’ll need to use OS X Internet Recovery to download Lion from Apple’s servers and install it on the erased disk. Whichever version of OS X you now have, run Software Update to get it up to date in that version; you will then be able to redownload Mavericks from the Mac App Store and reïnstall Mavericks, after which you can run Software Update to get it up to date.

If it gives you a hard time, the disk has probably failed, and so you will have to take the other fork and buy a new drive and an external enclosure to put it in while you install a new copy of mac OS X on it.

Only when it is working to your satisfaction in the external enclosure should you attempt to transplant it into your MacBook.

Frankiews98,

if your MacBook Pro is an Early 2011 model and it originally came with Snow Leopard, you could boot from its grey Mac OS X Install DVD and run Disk Utility on it to erase your internal disk.

If you have access to a bootable external disk, you could boot from the external disk and run Disk Utility on it to erase your internal disk.

If you have access to a second Mac and a FireWire 800 cable, you could boot your Mac into Target Disk mode, which will make it act like an external disk to the second Mac. You could then connect your MacBook Pro to the other Mac with the FireWire 800 cable and run Disk Utility on the second Mac to erase your internal disk.

Otherwise, you’ll need to take your MacBook Pro to an Apple store and have them do the wiping for you.