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.
The first thing I would check is to make sure your computer is recognizing your sound card. Go into Device Manager and make sure it is listed there under sound controllers and also make sure there are no yellow exclamation points or question marks next to the device name. If so then there could be a hardware or driver conflict.
If your computer uses onboard sound which is built into the motherboard then make sure it is enabled in the BIOS. If you are using an actual PCI soundcard then the onboard audio should be disabled.
You can also check to make sure the Windows Audio service is started. Click on Start, Run, and type in services.msc. Scroll down to Windows Audio. Double click it and click on Properties. Make sure that Startup type is set to automatic and that the service is started.
Finally I would try to reinstall the sound driver. If your computer is a name brand such as a HP then you can look up the model online and download the appropriate driver.