iPod Bought one song on iPhone and it overwrote whole library of songs

I had 4,000 songs last night on itunes and this morning I have one. Downloaded a song on the iPhone and synced. I will be reviewing my external hard drives to determine if I backed it up. I have backed up all tv shows and movies. Anyone know what happened? Need to avoid this ever happening again.

I have 2 apple tv's. One Ive purchased several shows. It is now deleting them. No idea what is going on. I've turned off iTunes to stop this madness. Will apple allow me to reload these iTunes purchases?