More info please.
a) what model of Mac?
b) what OS version?
By "stuck in a bonjouring state" do you mean the second splash screen with the progress bar part-way across the bottom? If yes, then the OS has encountered some problem, maybe corrupted, files. Either that, or you have a hard disk or boot sector problem. 20 minutes is an awfully long time.
Boot your Mac from Install disk 1 (hold the C key on startup), open Disk Utility > First Aid > Verify Disk - and then repair if necessary.
Reboot with the Install disk again, and from Disk Utility, Repair permissions. In fact, I'd recommend using OnyX Titanium Software to do OS maintenance regularly. I think it does a better job than the MacOS utility.