Will the water be able to drain out of a blocked toilet?
Water usually seeps past most blockages, but not always.  

You may have to hand bail the bowl down 1/3 or 1/2.  THEN employ a plunger.  The older cup plungers worked best with a strong UP pull (hence the bowl draining).

If its a newer, smaller tank, an older cup plunger may not make a tight fit:



This style may work better and will both push and pull against the blockage: