Somebody else posted the same error with the full error report (always post the complete error message, there is useful information in it) and his problem seemed to be, that a file called "collections.jar" was missing in his LimeWire folder. - This might be a problem with the installer. Anyway, the file didn't change since at least LimeWire 1.6 and you could always copy the collections.jar of a different version into your LimeWire folder.

Another possibility is, that the classpath in your start script is not complete (only on Linux and MacOSX). In that case it would be helpful if you could post the contents of your RunLime.sh file (or whatever the file you execute to start LimeWire is called) here.