battery icone disappeared in menu bar..my macbook big sur is not charging.can anyone help

Batteries are considered Consumable Products. Over time it will degrade to a point where it needs to be replaced. In you case - the usage is normal as the capacity will go up and down all the time and never stay always at 100%. Apple Batteries are rated for 1000 Full Battery Cycles and / or 80% Capacity before needing Evaluation or replacement.

For additional reading on Battery and Apple Computers this link may help. About battery health management in Mac notebooks and especially for the M1 and Big Sur If battery charging is paused or on hold on your Mac

You can try to bring the icon back to the menu bar by:

Go to system preferences > click on the battery option > Click on the battery tab > check the box which says "show battery status in menu bar"