I have a dell dimension 8400 (very old) worked great, then after some time not used I got a 'system battery voltage low' I changed the battery now it won't reboot. I get for long beeps short pause 2 more long beeps. I tried repeatedly hitting f2 to get to set-up to reset bios but it won't go into set-up
Depends on the BIOS, ie Award, Phoenix, AMI..
1) Check the manual to see what BIOS you Have, then Google "Computer Beep Codes" and look for the That BIOS.
ie if Phoenix it probably indicates a memory (Ram) problem.
If the Computer has sat for some time. The first thing I would do is to reseat every thing. NOTE turn the PSU off at the back (little rocker switch) leave plugged in
IE. Remove Ram and re-inset do this a couple of times for each Rame stick.
Unplug and replug your video card a couple of times.
Disconnect and reconnect all Power connectores from the PSU to the MB.
For HDDs and DVD drives, Demate and remate the Data connectors (both at the MB and the drive), the and power plugs
Reason, the connectors may have an oxidized coating acting as an Insulator.