Well some of these things are pretty obvious and I'm not saying you're stupid, so if they seem like obvious ones don't get mad at me, I just wnat to check if you might have forgotten to check them.
1.Is it plugged in fully? (really obvious)
2.Did you spill any water on either of the devices? If you are not sure, plug in a new set of keyboard/mouse and see if those work, if not, then thats not the problem.
3.Is you power supply and button cables plugged in correctly, sometimes the computer does not boot correctly with all devices turned on when some of those things are plugged in too loosely or crooked.
4.Did you mess with the BIOS before this happened? If so go to 4a.
4a.Clear BIOS, usually does this if you leave power off for 10 minutes or you set the jumpers to the right positions then reboot (remember to turn off power supply when doing this).(Only do this as your last resort)
5.I expirienced this and it is the PS/2 plugs have the little wires crooked, look at the little gold pins in your PS/2 plug and see if they are crooked or bent, fix them using your fingers(carefully) and see if it works now.
G. Skill 4GB(2x2GB) DDR800
eVGA 8800GTS 320MB
super pi: 1m=20s