It's difficult to answer because you haven't left any details. Are you using linux, windows 95, mac, ??? If you're using windows, try steps 1-3 here: Fixes for Limewire

If you're using mac, reinstall Java from here http://www.apple.com/downloads/macos...update142.html
& delete you LW prefs found here: /Users/your_username/Library/Preferences/LimeWire