keyboard stopped typing

My husband was just using his powerbook G4. He was trying to type something into the google field to search and said nothing was happening. The mouse was still functioning, so he tried to close it and pull up something else -- Yahoo and log in. Still couldn't get it to work. The keys depress but nothing came up. Then all of a sudden when he was typing alpha characters, numbers were filling the fields.

He restarted his computer and now it seems to be functioning normally. WHAT HAPPENED? And how to avoid it again?