My dell beeps six times and doesn't diplays - Dell OptiPlex GX280 PC Desktop

Hello,
Look at this article, (scroll until the down page), you will find a table of beeps and their meanings : http://en.kioskea.net/pc/bios.php3
Hope this will help you.
Best regards.

Listen carefully to the beeps and note the pattern. There won't be just 6 beeps in a row (unless the motherboard is very sick and needs replacing). There will be two or three groups of beeps separated by pauses. The code tells you what problem the motherboard ran into while trying to start. There are also 4 indicator LEDs on the rear that light either yellow or green and can give more information. A list of the beep codes and light patterns is available here.

Common problems are memory errors or troubles with on-board video. If this doesn't help you work out the trouble, please post a follow-up comment with the beep code you're hearing and I try to be of more assistance.