Try these...
CMOS battery failure. Will not stop PC booting, but will show the message
"CMOS defaults loaded. Press F1 or Del to continue"
RAM failure beeps
1 long beep followed by short pause then long beep again
will continue to do that untill you turn off PC. Remove RAM clean contacts and reseat. If there is more than one stick of RAM put one in at a time. That test does not always work depending on age and type of motherboard.
VGA Fault.
3 short beeps then 1 long beep, in rapid succession, then continues to repeat. If its an old PC this will either be loose or faulty card or monitor not connected. If youngish PC then will not beep if monitor not connected. Remove VGA card clean contacts and reseat
Tell us what the actual beep sound is, is it regular, how many, if its errattic.....
Mick