New graphics card giving trouble :(

I recently returned my old nvidia 6600gt for a new one.
I got the new card today and put it in my computer.
Everything seems to work fine but the monitor is showing no signal.
I tested out my monitor on another computer and it ran fine.
I tested another monitor on my computer and it has the same no signal problem.
Before I returned my card, everything worked fine.
I haven't touched my computer since.
Could someone help me?

 

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