First check the physical connection of speakers.
Most of the computers have multiple sound pin sockets like front and rare. So, check both if you have. Also try any other speaker or head phone if you can get temporarily.
If all above does not work. Try to reinstall sound driver.
If the system has built-in Sound Card,
go to run
type dxdiag and Press enter
Check the System Model
and then google driver for the same system model.
If you have any Sound card installed on your system, then you will have to find the driver for that specific Sound card.http://www.asknetinfo.com/intel-dg31pr-realtek-audio-driver-installation.html