Front brake won't respond until fully engaged
Work the lever and check that that the caliper is moving - that will tell you that the cable is attached and has not broken.
Make sure the little lever on the caliper that spreads the pads to let the wheel come out of the bike isn't engaged
Inspect the cable run from lever to caliper, make sure the housing is properly inserted into the lever and all frame stops.
If that all checks out the cable may have slipped through the pinch bolt on the caliper. Undo the pinch bolt to free the cable, verify the lever is pulling cable by pulling on the cable and and working the lever, reattach the cable. You can refer to this Park Tool video for installing and adjusting calipers.