We’ve all been there. You sit down at the computer, ready to undertake some arduous task (that Facebook page isn’t going to update itself, you know), and your mouse isn’t working. Try these easy troubleshooters to see if they fix your issue.
Replace the batteries. Yes, it seems simple, but you would be surprised at the number of people who don't think to try this first. Swap out those AAs or AAAs for a new set, especially if you're still using the batteries that came with the device. Likewise, make sure the batteries are properly installed. Sometimes closing that door before the battery bounces out can be tricky.
Clean your mouse. If the pointer is moving jerkily or is less responsible than usual, clean your mouse to see if it improves the performance. Regular mouse maintenance is something you should do anyway.
Try a different USB port. There may be a problem with the one you're using, so unplug your mouse or the receiver and try an alternate one.
Connect the mouse directly to the USB port if you're using a multi-card reader. There may be an issue with that device instead of the mouse or USB port.
Make sure you're mousing on an appropriate surface. Some mice can be used on (almost) any time of surface. Many can't -- know your device's limitations, and make sure you're working on the right surface. This may mean that you require a mouse pad, especially if you're using an older mouse.
Check the manufacturer's website for a driver. If your mouse won't do something the manufacturer promised it would do (side-to-side scrolling comes to mind), check their website to see if a driver is required. These are almost always free.
If you're using a Bluetooth mouse, make sure it's been paired correctly.
If your mouse won't click anymore because it's been worn down, check out Instructables.com cool fix using ordinary household items.
If none of these tips work, contact the manufacturer. You could have a defective cord, receiver or device. Whether it's defective or simply old and requiring a replacement will vary depending on the company's definitions of defective … and old.