First of all yes this is a serious problem. Make sure that your doctors take you seriously on this one.

The problem is caused by pressure on the nerve that supplies the muscle on the front of your shin. This muscle (named tibialis anterior) lifts (or dorsiflexes) your foot to prevent your toes from scraping on the ground as you walk. A lower back problem such as a lumbar(or lower back) herniated disc (probably the most likely cause considering your age) which is pressing on the L4 nerve root could be the problem. If you are active in sporting activities a blow to the outside of the leg just below the knee may have damaged the common peroneal nerve which supplies the tibialis anterior muscle.

If addressed soon and pressure is relieved the nerve can regenerate and function will return but if you procrastinate and just hope that the problem goes away you may have a "floppy foot" for life.

Physio may be of benefit to you but if you don't see any improvement in a few visits, over the span of 1-2 weeks, surgery may be the answer (assuming that the problem is caused by a ruptured disc in the lower back).

Good luck!