Having trouble booting my newly built pc!?

After turning on the computer, it shows the bios thingy (where i can press del, f2, f8 or f11) then goes to a black screen with a flashing underscore.

I just built this pc, parts in it are:

Intel Core i5 2500K

ASRock Z68 Extreme3 Gen3 Motherboard

Corsair Vengeance CMZ8GX3M2A1600C9R 8GB (2x4GB) DDR3

Gigabyte GeForce GTX 550 Ti 1GB Overclocked

Silverstone Strider 500W ST50F-ES

Seagate Barracuda Green 2TB ST2000DL003

Intel 320 Series 80GB SSD

I’ve tried booting it seperately with the ssd and the hdd, same result.
No cd/dvd drive, i’m intending on installing the os from usb.