Exclamation mark next to songs in my playlist

The exclamation mark means that the songs are no longer on your computer where iTunes is expecting to find them (if you click on them to try and play them do you get a message about not being able to find them and would you like to locate them ?) - have you deleted, renamed or move your music on that computer ?

If you can't find them on that computer then copy them back from your backup of your downloads/library to where iTunes is looking for them. Or if they are iTunes purchases, and they are still in the store and you haven't moved countries since buying them, then you might (after deleting their missing entries from the My Music part of your library) be able to redownload them via the Purchased link under Quick Links on the right-hand side of the iTunes store homepage