The source you were originally downloading from may not be online now.

Don't forget to search at different times of day, as users in other time zones log on, available files change.

You could try changing some of your ultrapeers, this will often give you different search results. Click on the Connections tab, right click on an ultrapeer > remove. A new ultrapeer will take the place of the old one. Try this with a couple & see if your search results change.