An arbitrary selection of piano keys are muted. Please help.

Dear All,

My problem is this: When I play midi instruments on my piano, a random set of keys are suddenly muted.

I use Logic Pro 9, and I'm a rookie. Hence, I'm not sure whether this is a bug or merely some extremely weird adjustment I've managed to do. I've checked the piano roll (it works splendidly); I've checked the mute tool; I've tried opening a new project; I've tried "reset all media regions"; I've tried restarting the program; I've tried restarting the computer. Nothing works.

Perhaps I missed something, but I don't know what that would be. This I know: All keys sound on Digital Performer; and after I've restarded Logic, all keys sound - for a few seconds. Then quiet.

After my first two restarts, I received a window, asking me to reset all media regions (which I did to no avail). Perhaps this window identified the error, but it won't come up any longer, so I can't quote the whole thing.

If anyone has any clue, please help me. I'm quite desperate.

Thanks in advance,

John