http://www.helpwithpcs.com/upgrading/installing_dimm_memory_ram.htmReference that article. If you have multiple modules, remove all but the one in the lowest numbered slot (printed on the motherboard are usually slot numbers like 0,1,2,3 1,2,3,4 or A,B,C,D)
If it still doesn't work replace the module in the lowest numbered slot with one of the ones you removed. (Having a total of one module installed.)
If you only have one module, take it out and put it back in. If that doesn't work try another slot.
If your system boots at any point you can use the process of elimination to determine the bad module.