[doctom54]

The two most common causes of foot drop are compression of the peroneal nerve and an L5 radiculopathy.
The peroneal nerve runs around the fibular head (outside edge lower part of the knee). It can be damaged there by compression (i.e. a bandage) or by direct trauma. The later is so called "Bellhop's knee" from the suitcases hitting the lateral aspect of the knee while being carried.
L5 radiculopathy is where the L5 nerve root, as it comes off the spinal cord, on the affected side is compressed. Either a herinated disc (HNP) or degenerative arthritis are the two most common causes of this.
Unless it is obvious (such as known trauma at either spot) the work up to determine the cause is a "little" involved. Generally, you need a nerve conduction study and an MRI.
Once you pin point the cause then you can start a treatment. Generally with physical therapy the foot drop will resolve, if the pressure is taken off the nerve soon enough.
If for some reason it isn't treatable then an AFO (ankle foot orthosis) is very helpful. This is a thin sheet of plastic that goes in your shoe and up the calf which holds your foot up and keeps it from flopping when you walk.
Good luck and lets us know how it works out.