You might have to mess with the big fork stem nut but usually that isn't necessary.
When a bike is layed down, often the handlebar will hit the ground and try to turn the wheel to the side. Then, the wheel hits the ground and refuses to turn which results in the fork tubes moving a small bit.
This results in the handlebars not being centered when the wheel/tire is pointed ahead. It does not usually result in the wheel not being able to be turned.
I know this is going to sound rather crude but the way many fix this misalignment condition is to straddle the front wheel with it between your knees.
Then, grab the handlebars and turn them hard in a direction that will result in them being straight when the wheel is pointed straight ahead.
If the alignment of the bars/wheel is very far out of position, some have been known to pull the motorcycle up to some fixed thing like a tree or building. Then, with the wheel located so it will hit the fixed thing if the handlebars try to turn it in that direction, turn the wheel away from the object with the handlebars. Then, rotate the handlebars rapidly as possible to cause the wheel to whack the fixed object.
This usually fixes it if luck is with you. The catch is, getting just the right amount of "whack" to correct the problem without knocking the wheel so far that it is now pointed in the other direction.
Usually the cause of a wheel not turning is something like the fender (mud guard) got knocked to the side and it is now rubbing on the tire.
If this is the problem, loosen the bolts that hold the fender and re position it so it isn't rubbing on the tire.