How restore original shape currently with cutout

To visualize my problem, envision a cylinder.

the cylinder was originally created with a rectangular cutout much like one would have if the cylinder were created on a wall displaced into the cylinder.

Later in the design, I decide to remove the cutout so that I end up with a plain cylinder.

Is there a way I can easily restore the original shape?

Well, with a cylinder, anyway, I would just delete all of the cylinder except one end and then use Push/Pull on the remaining face and extrude it again.

Love your response.

Thank you for taking the time to respond to me.

Have a wonderful day