Battery

If battery charging is paused or on hold on your Mac . If this is the issue it is because Apple has Coded into Big Sur an algorithm to pause the charging process so as to not charge to 100% all the time. It is purported there to minimize the Full Battery Cycles and thusly extend the life of the battery.

You can manually over-ride this if Battery is on the Menu Bar. Click on the battery Icon and see if it shows Paused. If so click and start charging. This will allow it to go above 80% but will Pause again once 90% is arched. Repeat the process to over-ride and it will reach 100%

What model exactly?

Assuming the power outlet has power and the power adapter hasn’t failed, try resetting SMC.