You can try starting up in safe mode by holding down the shift key. You may also want to start in single user mode, hold the S key down on boot. Run fsck -yf to fix any disk problems that may be present.
Whatever you do, don't erase the drive. You could reinstall the OS without losing any data. If that is not an option, and another Mac is available, you could use Firewire target disk mode to recover data.