New Purchases Disappeared in iTunes

I've upgraded to 10.5.1 without any problems until this morning.

I had a hard drive with a few thousand songs, songs that I purchased through iTunes on another computer, as well as songs that I ripped from CDs.

I just went in to play an album that I downloaded yesterday on my MacBook Pro and suddenly iTunes tells me that it can't locate the song. Same with an album that I downloaded from the laptop two days before.

It's maddening, to say the least, as the songs played just fine when I bought them yesterday. I click on the "Locate" button in the error message and enter the artist's name. iTunes displays a list of music from that artist, but the album that I bought yesterday is now greyed out. Again, played great yesterday on this very same laptop, and now these songs are inaccessible to me.

At least the laptop shows the songs, even though they're greyed out, but what could have happened to suddenly remove my access to these songs? When I enter a song title in the search bar in the finder, it says the file doesn't exist, although iTunes clearly recognizes that it's there- it just won't let me get to it.

Other albums that I've purchased on this laptop are playing fine- it appears to be the two albums I most recently purchased.

Any ideas? Any help would be greatly appreciated!