I have a power mac g3 it boots fine but mouse and key board do not work

Question

The mac boots fine, I can see desk top, etc. The key board and mouse are connected but do not appear to work.  This has just happened.

Answer 1

You could take it to a service center but: 1) It would probably cost more than the computer is worth. 2) You'd be lucky to find somebody who knows what a G4 is (maybe I exaggerate but really, they would laiugh at anything that "old"). 3) I don't think we're done here.
Do you still have the original discs. One of them has a hardware test that you can run.  You could also see
if it boots to the original disc and if things still won't work.  You could even try OS9.
Also look into CUDA reset and NVRAM reset.  I think they are both references in the link I provided earlier.
[X-Lab: The Apple Hardware Test|http://www.thexlab.com/faqs/aht.html]
[Apple's AHT support|http://www.info.apple.com/support/aht.html] - Apple Hardware Test Images and Information (for pre-Intel Macs)
Here's the list and downloads for AHT if you have an old G4...
http://www.info.apple.com/support/aht.html
If it really turns out to be the USB hardware that's bad you could probably find a third party USB card you could slip in there for next to nothing and would also give you USB2 speeds.  There's minor disadvantages.
Oh, did we determine if it really isn't working at any stage?  What about if you hold down the mouse key while booting the computer? That should open the optical drive door if it is being sensed.