Just take any working bootable external Mac disk (Firewire is my favorite, flash drive is fine, USB works too in some cases - not to forget Thunderbolt) and reboot your MacBook Pro from the external disk. To boot from an external disk you'll need to hold the option key down while starting up.
Now that you're up and running again:
- copy any files off the boot drive which you don't have backed up.
- do a clean install of the OS
- test that you can boot now.
- copy back any missing data files
- reinstall any applications which aren't working or have gone missing.
Enjoy troublefree computing again. Be happy that Apple still allows booting from external drives (and usually from other computers) unlike those poor souls over in the Windows world who can only boot from an install disk.