Disc brakes stop working after a wash
As @Carel mentioned, try using rubbing alcohol to clean the discs. It's often called Isopropyl Alcohol or isopropanol.

You may have inadvertently contaminated your pads which often means replacements. Did you use any sprays which may have blown on to the disc? Frame protector and water displacers like gt-85 or wd-40 will play havoc with your braking power.

I suggest taking the pads out and keying the surface with some emery paper. It'll take a little while to re-bed them in but it might bring them back to life.

If a contaminant has soaked in to the pad too deeply though you'll need to swap them out.

You may also want to take the opportunity to re-centre the calliper whilst you're tinkering.