On the old design taps that are basically a screw that moves a washer up & down, then washer could be stuck, - renew it.
If it is more modern design then it could still be washer, but also could be works disconnected. As said, check circlip, but could be the common problem of wear on the O rings inside the works, that means water enters into the part the threaded bit goes. This is often on hot taps due to heat. Remove the works, take clip off, ( the works are in 3 parts), tap spindle out, screw spindle apart, & as you probably won't be able to match the O rings exactly, just clean & grease with Vaseline or silicone grease, & put together & refit. May last reasonably well. If you could get a new spindle with same splines to suit tap head, then often better a new one.