I was having the same problem. It just started one day and I tried everything -- trashing files, resetting, emptying cache, reinstalling, etc. All to no avail. Then I started reading these forums and something clicked when someone mentioned they were using Earthlink and had to reset their DNS numbers. It reminded me that i had switched dial-up access numbers trying to get a faster connection. So, I just chose a different dialup number and it's working again!
I doubt this answer would work for everyone with this problem, but thought I might mention it incase it helped someone else.