When I do start > run \\servername I used to be able to see all subsites and libraries. Now I get a blank windows explorer window. My sysadmin claims he didn't do anything. Any ideas?

did any error you see in logs files? I have this kind of problem but not exact scenario, little different, In that case permission was distrubed and i reset the permision then user able to see the explorer window. are sharepoint site is mapped to the drive? hope this will help thanks wsi am at SharePoint administrator