FRUSTRATED! Can't connect to server.

I have a Windows PC which I use for a file and print server. Everything worked fine until a couple days ago when all of a sudden I could not access the server anymore. I can still print fine on it, but cannot connect to any shared volumes. The server shows up in the sidebar and I can click on it, but it always says connection failed. If I go to Connect to Server and enter the IP or the name of the server, it asks for my name and password but will not accept them. It tells me that I do not have permission to access this server. I know the name and password are correct. What could be happening here? On the Mac side I am using a MacBook Pro with OS 10.5.2. Thanks!

Oh and if it matters, the PC is running Windows XP Pro with SP2.