Direct impact to medial side of left ankle from slipping on wet stairs. x-ray does not show fracture, however foot doesn't plantar flex. ?

Don't wait.: I suggest you see a foot and ankle specialist ASAP. A directed ankle evaluation is needed to determine the nature of the lack of plantiflexion. The X-rays only show possible bony damage. Lack of motion can be from bone, muscle and tendon or even edema. The range of motion of the ankle joint needs to be assessed and this is best accomplished by a specialist that familiar with the anatomy.