If you've ever tried to unroll a projector screen only to find that it appears to be stuck or locked, what you've actually done is enabled a safety feature of the unit. Just like a car seat belt, a projector screen will lock up and prevent itself from completely unrolling if you start to pull on it too fast. This is to prevent it from just randomly falling on its own. To fix this, you just have to unroll it the correct way.
Pull down gently on the bottom of the projector screen. You should feel it start to retract itself up into the screen storage container.
Let the screen retract itself all the way up into its storage container. When it's in its fully closed position, you should only see the little metal (or plastic, depending on the unit) grip on the end of the screen and nothing else.
Pull down slowly on the grip to unroll your projector screen. Don't pull down too fast or it will lock up again. When it fully unrolls, the projector screen will lock itself into place.
If your projector screen still will not unroll, something may have got caught in its storage container. Lift the screen down off the wall where it is mounted and set it on a flat surface. Use a screwdriver to remove the screws on the top of the compartment (this will change depending on brand, model and size of projector screen). This will allow you to open the compartment. Look inside and remove any obstructions from both the part where the screen is stored and the locking mechanism. Seal the screen back up and mount it again.