My MacBook Pro will boot up but won't login
If the Macbook boots fully but will not log in it is likely that there is something that starts up in that user profile that is preventing boot up. Wht you need is Safe Mode.

Boot the Mac while holding down the shift key. When you see the Apple logo come up you can release the shift key. The Mac will boot, I have seen booting in Safe Mode take a long time. This is because Mac O/S does a bunch of maintenance routines during safe mode, including disk and filesystem repair, so give it time.

Once you are at the login screen I would bet that you can now log in. Now back up your important stuff just in case and try turning off items that run at startup.