Sounds like you have glazed your disc and pads, leaving the wheel on your bike get some fine sandpaper and lightly pinch the disc and spin in the normal rotation of the disc add a small amount of water and keep spinning until the water goes a dark gray, repeat until your happy that the shine has gone.
Then get a clean rag and wipe of the gray water, repeat again but with brake cleaner let it dry, wipe of what's left and you should be done for the disc.
Muc-off brake clean should do the job or clutch an pad cleaner for motorbikes
For you your pads do the same pretty much the same thing lightly sand with water until the pad goes dull, rinse of with warm water then let them dry.
When your braking the pads back in take it easy on them no sharp braking or holding on brake well as riding as fast as you can you don't want the disc getting hot, this will glaze again. Just ride at a steady pace and apply the brake until you stop, do it a few times and they should start biting
If this happens again it could be down to the pad being worn if so replace the pads and repeat the above
I had the same problem not so long ago and this done the trick perfectly.
Hope this helps
Scratch all of the above