Go to Control Panel > System and Security > Power Options and check that your power options aren't affecting the brightness on your screen. While there, check that the brightness isn't automatically adjusted by your PC or laptop. Update your display drivers
You can fix the brightness problem by using the below methods:
Method 1. Update your Windows 10 Operating System.
Method 2. Disable/Enable your Display Adapter.
Method 3. Update your Display Adapter.
Method 4. Reinstall your Display Driver
Method 5. Update Drivers Manually.
This is one of the most common problem in Windows. Low or High brightness often leads in disrupting the overall computer system. Read this blog to know the complete process - https://wethegeek.com/brightness-not-working-on-windows/