About half of my iTunes library doesn't work after syncing 4S and iPad 2

My wife and I both bought the 4S and after syncing with my iMac (quad core with i7- but not using Lion yet) we noticed that about half of our iTunes library has an explanation point to the left of the song and it won't play. The songs that do not play appear to be the ones I purchased on the previous computer (HP). All songs purchased on the current iMac and songs downloaded via CD all seem to work. Any suggestions on how to fix this? Thanks in advance!

Nevermind, I just restarted the computer and everything is back to normal.