Disable button and renable button from

I have a button that is disabled after opening an external loaded movie. if that movie is unloaded, how do i re-enable that original button?

Senario:
Button “A” is pressed
Movie “A” is loaded
Button “A” is disabled.

Button “B” is pressed
Movie “A” is unloaded.
How can Button “A” become enabled again.

thanks.