What does your Device Manager say? Also, do you have an integrated motherboard sound card or a seperate sound card in a PCI slot? If you have the latter, and as you have been cleaning the inside of your computer, it may be worth reseating your sound card.
Other things that spring to mind are the basics, e.g. mute, sound on low, speakers plugged in and switched on, etc. It's suprising how often the basics can catch you out.
Take a look at this link
which walks you through some of the sound card troubleshooting tasks.
Hope this helps.