Check cables connected correctly. There are quite a few connections on pcs and it's easy to get them mixed up! Make sure they are pushed correctly as well.
Assuming that all your connections are OK look in the space next to the clock in the bottom right hand corner of your desktop. There should be a speaker icon. Right click on this and then click on Playback Devices. A new window will open with a few different speaker options. One of these will probably be checked. Uncheck this and click on another to try it. (and so on until you find one that works)