Cisco router issues

i have several cisco 1721 series routers that appear to have lost their IOS, but i am not sure what exactly is wrong. Some CCNA students were working on them, and apparently corrupted something...one of them doesnt respond to a console connection whatsoever, and a few of them respond, but with characters that make no sense whatsoever. Does anyone here know how to restore the IOS when you can not configure anything at all??

I do have a working router to copy the IOS from, but would need step by step instructions due to the fact that i am still not very familiar with router configuring.