Power-On Self-Test (POST) beeps would be a good thing to check, as smalloy says.
Assuming you didn't touch anything else other than the memory, (which you may well have done, if the memory was difficult to get at without taking the machine apart), then there are three main possibilities: the memory you put in is bad, the memory you switched back to wasn't put in right, or you cracked the mainboard pushing it in wrongly or just too firmly.
However, if you did take other parts out, just start from scratch, building the machine again with just the essentials, and then try the other stuff, one bit at a time.
Also, make sure you haven't just knocked the power cable out or hit the off switch on the PSU or something (if it has one)