there's a lot of things that'll cause nothing to be shown on the crt.
but the single beep indicates that your video card was seen by the bios.

wrong or bad memory can cause this.. as well as a problem with other
cards you might have inserted.. or your power supply isn't good enuf to
handle the needed current draw.. expecially if you got a high end video card.
better tell us a few specs/ratings/manufacturer of power supply, mem, video.. mobo.

you have to strip it to it's basics.. no hard drives/floppy/cdrom and their cables attached. only your psu, memory.. and a cheap video card to start out with.. and work up from there if you start seeing a screen.