Why did I lose braking power after hitting a curb? Recently I hit a curb on my bike. I heard a cracking sound, my front disk brake (cable) felt stiff, and I could feel a lot of resistance from the front wheel. I left it alone for a few minutes, and when I came back, the brake was back to normal, but I could hear some grinding in the front bearings. I also soon realized that I couldn't stop as quickly as before. Today I took the bearing apart, cleaned it, applied fresh grease and the grinding disappeared. The brakes still were noticeably weaker. I tried smoothing the rotor with sandpaper, but that did not help. What could be the reason?
Additional information:
I was moving at around 25 km/h, the curb was about 3-5 cm
The axle is straight, the dropouts seem to be fine
The cable shroud seems to be a bit deformed, but that deformation does not change how far it can retract
I took the whole caliper apart, sanded the rotor and pads. After reassembling, I think it is a little better, but not as much as it was before. Or that's just placebo, I'm not sure.
Based on the second photo, I'd guess the rotor is no longer parallel to the brake caliper.
Thus instead of having flat contact between the pads and the rotor like
=| |x| |=

you now have smaller area of contact.
=/ |x| /=

Sanding the pads has rounded them off a little, increasing the area of contact a little.

The best fix I can come up with is re-aligning the caliper with the rotor.

Take out the front wheel and fit a slip of paper over the rotor, then reinstall so the paper is in the caliper.
Back off both bolts holding the caliper to the fork, enough so it can slide.
Press and hold the brake lever on the handlebars and help the caliper find its center.
Tighten the two bolts sufficiently (not excessively)
Release the brake lever
Spin the wheel to pop the paper out, and test.  You're looking for no-rubbing and that the pads touch the rotor all-at-once.

Small changes will have a big effect on the brakes.
The problem may be if the little mounts that support the caliper have been bent, or if the caliper's mounting tabs have bent, or if the mounting bolts are bent.
You might need to replace both bolts if they are not straight.
I would not recommend attempting to straighten the caliper if its bent - they will probably break, or if not will be weakened.
If the fork's mount points are bent, you may be able to file/mill/sand the top surfaces to be flat and at right angles again, but it is hard to get perfect.
Upshot - avoid riding your bike into kerbs/curbs.