If they are power amplified Make sure they are plugged in
and powered.
Make sure they are connected to the correct spot of the
sound card. With Some sound blasters and other 5.1 4.1
Etc. Cards it is difficult to decypher Which is for the
Main speakers and which is for the backup
Also try plugging another pair of speakers into the
Computer and see if they work correctly at all..
If it is on-board sound, make sure it is enabled in the
BIOS
If it is a sound card, or external sound box, try moving
it to another location on the machine (I.E. If its USB try
moving it to another USB port, If a PCI card, move it to
another slot)
Didnt give alot to go on so those are the best suggestions
i can muster..
If everything else is correct, Go to the manufacturer and
try updating and re-installing the drivers