My iMac refuses to log me in

Question

1. I'm not an idiot, caps lock is off, num lock is off, I am using the correct password.
2. When I type in an incorrect password, the screen shakes as it's supposed to.
3. When I type in the correct password, the screen changes as if it is about to log me in, but goes straight back to the login screen in a second or two.
4. I have gone into single user mode and created a brand new admin account. Same thing, doesn't log in.
5. Safe mode does not work either.
6. Guest account is the only thing that actually works.

I've been a PC user for almost 8 years, so I am relatively new to this whole Mac thing. The problem I am getting does not appear to be a very common issue, as I can only find a few threads on Google that relates to this.

Last resort would be to reformat the computer -- I really want to avoid this option since 1. I have not backed up any files, and 2. This is my best friend's Mac, which he lent to me while he is spending a year overseas.

On a sidenote: The iMac fell on its side yesterday (yikes!), but since the Mac boots up and goes to the login screen, I doubt I have damaged the hard drive. However, since I am new to this whole Mac thing, I'm not 100% sure if the fall was the culprit of this issue. Anyone care to chime in?

edit: Thanks for the replies. I booted the iMac with the Recovery Disk, and I tried re-installing the OS. The HD in which OSX was installed in does not show up -- so I tried deleting the partition. However, it gives me an error that says "Volume Erase Failed - Couldn't open disk." Does this mean my hard drive is done?

Answer 1

One thing to do would be to trick the iMac into thinking its a new setup but jumping into single user mode and deleting the .AppleSetupDone file in /var/db.

Restart the mac and it'll act like the machine has just been pulled out of its box. (all your data and accounts are still on there).

Create a new admin account and see if you can login. You should be able to. If not something is seriously broken. If you can, run the standard system checks to see if anything is amiss.

Answer 2

Sounds like something's screwed up on a system level. The guest account might only be working because it might be configured such that files don't need to persist: IE, it doesn't need to be writing files to the disk.

Backup your files, reformat, and start from scratch. It might be repairable still, but you'd need to spend a lot of time investigating and fixing, or you'd need to hire someone.

EDIT: Silly me, I didn't read it as thoroughly as I could have. Boot from an OS X disk, go past the first screen (careful -- don't wipe your data) until you see the Utilities menu. You want Disk Utility. Run a verify and perhaps a repair if it finds anything. Run a verify afterwards again to make sure problems are fixed.

You can also try DiskWarrior, which in my experience is more thorough (and capable) than Disk Utility, but isn't free.

Answer 3

Another solution is to start the computer up and hold down the option key. Then select the recovery HD as your start up drive. Wait for the computer to start up from it and then select Terminal. Simply enter the UNIX command that resets the password to whatever you want. This backdoor option is something to know about because it is also a BIG security threat to your computer UNLESS the computer has been locked down with a firmware password.