To make sure I'm understanding this correctly, your computer was running with no problems, you added one ram module to the computer and now it's not working.
The module you added, was it from the same brand as the original? Does it have the same timings as the original? Is the voltage the same as the original?
When upgrading ram, it is recommended that it is done as a kit which will have 'matched' modules. Here's where much of the problem lies... ram often does not play nice with other ram (sometimes even the same exact modules don't like each other which is why manufacturers match up "kits" which they have checked for compatibility).
On some systems there may be a way to work around this but you may, 1) lose dual channel functionality 2) have a slower system than you started with. It does involve getting into and adjusting settings in BIOS, perhaps a few times. Still no guarantee of it working but if you're game, we can (hopefully) get you going with the two modules.