|Your post is confusing. Did the problem occur after you installed the new graphics card? Maybe your computer can only use 2GB of RAM. |
Hard to say one way or the other when you have not provided any specs or model numbers.
Does the new card require an auxiliary power connector? If so, have you connected it? If so, your power supply may not have enough amperage on the 12V rail. Most pre-configured computers have marginal power supplies.
After checking the power requirements the power supply may not be the issue. You may have disturbed something inside the case.
I hope you unplugged the computer BEFORE doing anything inside. Re-seat all cards, cables and RAM. If you jarred the heat sink you probably need to remove it, clean off the old thermal material and apply thermal paste according to the instructions for your processor.