Do you keep getting a low beep- beep signal ?
I just had this problem and it took me 4 hours to figure out what it is.
My suggestion, First reset the BIOS.
Since the new stick of RAM maybe different from the ones you had inside earlier.
Now, add just One stick of the new RAM in Slot one and leave the rest empty.
Start the rig and see if it shows up in the BIOS. If it does , good, if not, remove that stick and put the other one in , boot and check again.