Did my video card just die?

Hi and help!!

I have a 3-ish year-old CQ3240AN running Windows 7 (64 bit). About 4 weeks ago I upgraded the graphics card to a Sapphire Radeon HD7730 1GB GDDR5. I also put in a new 550 W PSU to support the card (Corsair VS550) and another 2GB of RAM (Corsair 1333 MHz DDR3).

Everything has been working well until yesterday when my son turned the PC off and 10 minutes later my daughter turned it back on. Although the PSU seemed to start normally (lights on, normal start-up noises), both the monitor screen and the HDMI output to the tv remained black. I tried re-starting, but with the same result. All external cabling is in place and both the monitor and tv were turned on.

Any suggestions, please?

Thanks in advance.