If it's an iMac G3 or a G4 desktop, there's a Reset button on the motherboard to roll back all the POST and PRAM (Apple's Motorola-Mac counterpart to BIOS, roughly-speaking) to their factory defaults. iMac G3s are fiendishly tight inside, as their appearance might hint at, but with the G4 mini-towers it's often just a matter of moving a hard-drive ribbon cable out of the way and scanning the board with a small flashlight. Some models had pin-recessed Resets that responded well to a straightened-out paper clip.
There's also Open Firmware, situated on the ROMs of all but a few rare Mac models capable of running OS X (onto which OS X has been installed a bare minimum of ... once!) for which there are probably ample tutorials on various Mac-related sites as well as apple.com. It's comparable to Microsoft's Recovery Utility or Linux's GRUB or modprobe in the relative obscurity of its command library, but I have heard of numerous instances where hardware settings were at least temporarily restored by way of OF.
Intel Macs -- can't help you there.
To find out what kind of Mac this is, try one of these two sites:
Hope this was helpful.
15 years a Mac home user (5 "genuines" and one clone)
5 years in OS X (power user, never a tech)
1 year as a phone tech for an AASP