The single beep sound mean u have pass POST (Power On Self Test). Various beep sound means different problem.
No beep sound is the worst problem. U have FAILED POST. Likely your processor or the mobo is fried.
Had a similar problem b4, thought it was mobo. Changed and still the same, finally changed processor and sloved the problem.
Quite diff to test, it could the mobo that fried the processor, unless u have a spare old processor that u don't mind get fried.
A quick way to test if it's the RAM problem. Plug out the RAM and boot the system. If still no beep at all, then it's not the RAM problem. RAM problem will give u a few beep sounds depending on the bios.
can be a hdd failure too...
it's easier if u got other replacement parts to test
if not juz use this method
unplug the rest of the hardwares except cpu, mobo, psu, vga card and ram (try use only 1 if u got more than 1 ram)
monitor of course keep it on.
if can boot up, means those parts r all ok
then will be your hdd prob.
if not then need to find replacement alr. borrow from fren will be a gd idea as my fren always do tat