Yes, the leg could be paralyzed overnight. Wings could go next.
Marek's is a very "unusual" disease in that it can and cannot have a timescale. The bird could ONLY have one leg go down, one wing, or one eye.
Usually the bird dies because it cannot reach food or water (the leg being...
Have you ever heard of Marek's?
Wikipedia isn't my favorite site for this, but it is a good general starting point. It could also be that he slipped a tendon in his leg...
http://en.wikipedia.org/wiki/Marek%27s_disease