1. Restart the computer, sometimes this simple operation can solve the problem.
2. Check whether there is a problem with the computer driver. You can uninstall and reinstall the corresponding driver in Device Manager, or update the latest driver version.
3. Check whether there is a power management strategy that causes the brightness to be unable to be adjusted. You can enter power options to adjust.
4. Check the settings of related shortcut keys or trackpad. Perhaps these settings have been changed, so the brightness cannot be adjusted.
5. Manually adjust the brightness setting of the monitor. If you can't adjust the brightness directly in the operating system, you can enter the menu of the display to adjust it.
If the above methods can't solve the problem, it is recommended to consult Lenovo's official customer service center for more professional support and solutions.