Help! front wheel brake doesn't brake (Shimano)
Sounds obvious, but first ensure the cable is securely attached to the caliper by the pinch bolt.

Check that the brake housing that the front brake cable runs inside is properly fitted into the brake lever. If the housing has come loose the cable has nothing to pull against and will not operate the brake calipers. 

If you have v-brake calipers, check that the curved metal 'noodle' is properly fitted into the caliper. The noodle can be popped out to spread the calipers to allow removal of the wheel. 

It's also possible that the cable has snapped inside the housing, or the cable head has broken off.