Okay... Don't panic !!!
First, please post all the info on your computer type
Your Operating System
What p2p client (program) you are using
and you might as well mention your internet connection, even if it doesn't apply now !!!
Hopefully, you have not actually 'lost' anything...
p.s. No matter what, you will probably need to create a new playlist, anyway...
Maybe not, but prepare yourself at least for that one, now !
Please also include info on what 'player' you are using...