If a Mac computer does not recognize the mouse, first rule out any physical problems. Most of the time these involve a USB mouse plugged into a bad USB port or a wireless mouse running out of batteries. Other times the mouse needs to be replaced. Troubleshooting the problem determines whether the problem is a simple physical one, or a sign of a larger issue that requires a system restore.
Disconnect all other USB devices except for the mouse and the keyboard. If the red light underneath the mouse isn't on, plug your mouse into a different USB port. The USB port is a long, flat rectangle with a tab the plug fits into. A USB plug will only fit into a USB port one way, and it will easily slide in. If the mouse doesn't work with either of the USB ports on the keyboard, try each port on the computer. If you have another mouse, plug it in. If that mouse works and your old one doesn't, your old mouse needed to be replaced.
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Troubleshooting With Startup Manager
If the mouse doesn't light up, or lights up and doesn't move, reboot the computer. When you hear the startup tone, hold down the "Option" key. The computer will boot into the Startup Manager. Check for a red light underneath and move the mouse. If the mouse still doesn't move or has no red light, the mouse is likely dead and you will need a new one. If the mouse does move, then you will need to reinstall the software and operating system on your computer. When reinstalling, be sure to check "Archive and Install" so that you don't lose your files.
Wireless Bluetooth Mouse
If your mouse is wireless, first change the batteries on the mouse. Replace the old batteries with unused batteries. If the mouse still doesn't work, turn over the mouse. Check that the mouse is on. If the battery cover isn't on properly, the switch won't move. If your mouse is on and paired, the indicator light will be be on. If the light is blinking, the mouse needs to be paired with your keyboard. After five seconds, the Setup Assistant should appear and guide you through the process.
In the upper left-hand corner of your computer, a Bluetooth indicator looks like an angular "B." If the indicator is all black, it means that Bluetooth is working, but no devices are connected. Double-check that the keyboard and mouse are on and have batteries. If a battery flashes across the indicator, it means that one of your devices has low batteries. When the indicator is gray with a black wavy line across it, it means that Bluetooth is offline. Restart the computer to bring Bluetooth online. If Bluetooth is still offline, restart the computer in single-user mode by pressing "Command" and "S" at the same time after the startup tone. If Bluetooth and your mouse work in single-user mode, then you need to restore your operating system and software. If the mouse does not work in single-user mode, you need to replace the mouse.