If your scroll wheel stops working: first, check if it works in a different app — some apps intercept scroll events. Open a mouse test or a long webpage and try scrolling. If scroll works nowhere: clean the wheel by blowing compressed air around it, as dust is the most common cause. On Windows, update mouse drivers in Device Manager > Mice and other pointing devices > right-click > Update driver. Check Settings > Bluetooth & devices > Mouse > scroll settings and make sure scroll lines isn't set to zero. On Mac, check System Settings > Mouse > scroll direction settings. If scroll works erratically (jumping or reversing direction), the rotary encoder inside the wheel is likely failing from dust — cleaning usually fixes it. For Bluetooth mice, disconnect and reconnect. As a last resort, test the mouse on another computer to confirm hardware failure.