Try the new as the only DRAM, if no joy, try a hard reset and then try it (again as only stick) if still no joy, it's simply probably not compatible. If it will boot after the reset then reset again and try old and new
1| Can you please pass on the information of your prior stick of ram?
2| Have you made sure your BIOS was up to date before you added the new stick of ram?
3| Were you on a 32bit architecture of Windows 7? In fact what version of Windows 7 do you have? Starter?
Try the new as the only DRAM, if no joy, try a hard reset and then try it (again as only stick) if still no joy, it's simply probably not compatible. If it will boot after the reset then reset again and try old and new