Keyboard not functioning

The keyboard problem I faced yesterday was weird. I was reported that keyboard was not functioning anymore. I asked them to check the connection of keyboard and unplug and plug again the keyboard to the CPU if necessary. But after a few minutes, they called me again, that they were still having the problem.

Then I went there and saw that the keyboard was recognized and functioning till to the log on screen. Even password could be given, but whenever the desktop screen appeared, keyboard didn’t respond anymore. I couldn’t find any visible reason of not functioning the keyboard. I was baffled and then used system restore and it functioned perfectly. But still I don’t know the reason of such weird behaviour.