Video output is not detected after reset of initial boot

Hello, all!

I am working on a home build computer with a:
Ultra LSP 550W PSU
ASRock M3A770DE mobo
2x2 GB RAM
Sapphire Radeon 6850 HD graphics card
AMD Phenom II 955 processor

Here is my issue:

Having just built my first computer, I power it on and it boots up. However, I accidentally placed the mobo disk in the drive instead of my OS disk. It asks me if I want to create a SATA(?) diskette Y/N, and I find out that my keyboard does not work (this might sound ridiculous, but it's actually an apple keyboard that I placed in the USB drive just to experiment, I suppose that it shouldn't surprise me that it didn't work).

So I pressed the reset button, and my computer resets...

However:

Now, nothing shows up on my monitor. It says that 'video output' is not discovered, as though it was not hooked up right. I have no idea as to what to do right now, as nothing is different. Why, all of the sudden, is there no video output on my monitor?