Many XT and some AT-class computers beep once or twice when booting up normally. If your computer shows standard information on the screen, you do not have a problem; if there is anything wrong the computer will display a scree error message. If you have no video display, check the simple things first, Is the video monitor plugged in and turned on? Did the video cable from the monitor to computer become disconnected? This single beep tells you there is fault memory refresh circuitry on your motherboard. the timer chip told the DMA chip to go into RAM and refresh the memory. The DMA chip did this, but the refresh process failed. The possible causes of this malfunction are (1) bad memory chips, (2) a bad DMA chip, (3) bad memory addressing chips on the motherboard. Turn off the computer. Reset the memory chip or the SIMMs or DDRs, then retest the computer. Since the DMA chip is almost always soldered to the motherboard--as are the memory address logic chips--any problems with these chips almost always requires the replacement of the motherboard.