dell d620 laptop, suddenly mouse wont work, neither will mouse button

Are you talking about the trackpad?

If so, it's likely the trackpad has failed. Once thing you can check though is to make sure that the cable underneath the keyboard is secure since the trackpad is part of the keyboard. To do so, you notice a small piece of trim at the top of the keyboard. Start at one an and slowly and carefully lift it. It's just snapped in, though you'll want to be careful not to break the tabs. Once this is lifted off, you'll see that the keyboard is held in with three screws (if memory serves me), remove these. Then you can carefully lift the keyboard up and out. Be careful, it is attached by a cable. Have a look at the connector that the cable goes into on the motherboard. See if it looks like its straight. You could even reseat it by releasing it and pulling the cable out, seating back in the connector and locking it in place again. Once reassembled start it up and see if it works. If it does great, if not then the trackpad is done.

If you are unsure about doing this without a pictorial, here is a link:

Service Manual

It could be that the track pad has simply failed like explained above, or what happens a lot is there can be a button used to turn it off, literally. I am not sure if this is on Dell also, but on HP Laptops, in the upper left of the track pad there is a little curved indent, and if you double tap it with your finger the track pad is disabled but you can still use any USB attached device. Note that when this happens there is usually a little orange or red light that turns on near it. Also, your driver could have crashed or have been disabled. Look up how to reset device drivers and follow the instructions. Hope this helps, have a wonderful rest of your week!