This stupid thing won't start!

[kydieselgal]

On the way home from a trip I stopped to fill up my '85 300D and it wouldn't start afterward. I know it wasn't bad fuel b/c it had been doing this for a few weeks, but it always started after a few minutes. This time it wouldn't. It definatly wasn't the battery b/c I was able to roll up the windows and sunroof, but nothing else. The glow plug light came on, but when the key was turned it wouldn't even click. A friend that knows a little about cars said he didn't think it was my starter, maybe a relay switch. Does this sound right? And if so how much will it cost to fix? Help!