We need more details on the setup.  Wild is probably correct that it is a tree issue of some sort.  If it went from working to not working and you didn't change anything the problem is most likely environmental or poor initial setup.

How far from the tower are you?  How high is the tower and your radio? Is it clear line of site or are there trees and/or buildings in the way?  What brand of equipment are they using?  Have you tried connecting your PC directly to your WISPs radio (or the LAN port of their POE block)?