Are you connected properly (how many green connection bars do you see)?
If your connection is ok, try this: Problems with the search

And if that's no help, delete LW's preferences folder when LW's properly closed. See point 3 here & also point 2 if you've used older versions of LW in the past (there could be a folder in either location).
Fixes for LimeWire
This will kill all of your settings within options, you'll have to go back through afterwards & reset things.
LimeWire Pref Folder in Vista

In LW go tools > options > searching > basic & check whether the 'show license warning' box is ticked. If it does, untick it > Apply > ok...see if that makes any difference.

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