There are a couple of things we can look at here. The obvious things are to check the speakers and volume. If you know the speakers are plugged in and are working then we can bypass this step otherwise you may want to check them on another computer or try different speakers on your computer.
Next let's double click the speaker icon on the taskbar down by the clock. Make sure the volume is up and not muted.
Another thing to look at is to go to Control Panel and then the Sound icon. Make sure nothing is muted and that you have a sound device listed there with a green check mark. Also check that the right device is set as default.
You can also check Device Manager to make sure there are no conflicts. You can reinstall or update drivers from here if you have your software from your sound card. You may also want to try to rollback the driver to a previous version in case it got corrupted on an update.
The worst case scenario is that your sound card went bad. If its a PCI card type sound device you can take it out and replace it with a new one. They are pretty cheap nowadays ($25 and up). If your sound is built into your motherboard then you can disable the on board sound in the BIOS and add a PCI sound card.