Laptop powers up with dim screen

Post Windows 7 Home Premium to Windows 10 Home upgrade on my Acer Laptop, everything went reasonably OK apart from having to resize the Reserved System Partition.

After a couple of days, the brightness of the screen at start up is really dim making it difficult to see or do anything. I initially thought that Windows had crashed & did CTRL/ALT/DEL to restart.

The only thing that I can to is hold down the Fn key & press Screen Brightness key which seems to work.

But, why should it do this after a couple of days.

Not aware of having made any changes to initiate this.

Would really like it to boot up at full brightness.

Any suggestions gratefully received.

If you have "automatically adjust screen brightness" turned on, you may want to (temporarily) turn it off and manually adjust your screen brightness. The driver may need update for Windows 10.

This morning the Laptop failed to start just reaching the Windows 10 Logo with rotating dots that stopped.

Tried everything to repair, start up options in Safe Mode - even reverting back to Windows 7 but no option other than to restore to a Macrium Image I fortunately made before the upgrade.