Memory Slot Not Working

A while ago I upgraded my iMac EMC 2134 to 4 gb and all worked fine until one day when I switched it on there was only a black screen. Power was getting to the computer and I could hear the DVD drive start to work. I tried everything I could think of to get it to start even removing the new memory and replacing it with the hold. Nothing worked until I started the computer having forgot to put the memory in slot 0. As soon as any memory was placed in that slot the computer refused to boot. Has anyone else come across this problem and is there an easy solution to fixing it