Me too, many times! <grin>
Earlier tonight I was beating my head against the wall because I couldn't get sound from my speaker system and I was trying to watch Planet of the Apes (the reboot...which is terrible, for the record)...
Yeah...somehow I'd muted my system volume.
Derpy derpy derp derp...
I had a problem one time where I was on MS Word and I couldn't figure out why it kept spacing over. Turns out I had something sitting on the space bar.