Check your brake discs

Brake discs (rotors) should all have their minimum thickness stamped on them, the discs will wear over time and once they get down to the minimum thickness then they should be replaced.

Always check your discs for damage, and also for warping, usually just slowly spinning the wheel will show if the disc is warped (twisted out of shape) as you will either see it, or hear the disc catching on the brake pads - a better check though is to hold a pencil firmly against the fork leg and touching the disc, spin the wheel, if straight, the pencil will touch the disc all the way around.
If it's warped, you cannot repair it satisfactorily, replace it, a warped disc can easily just break whilst you're riding, as can a disc that's been allowed to wear too thin.
Cracks in the disc are extremely dangerous, they tell you that the disc could split at any time.

Have a look at the last picture here - the disc has warped /cracked whilst riding and it could not pass through the pads in the calliper, the result was that the front wheel locked solid immediately, damage is to the pads, calliper, disc, possibly forks.
The rider would have been thrown over the handlebars and the bike probably somersaulted.
Check your discs regularly for damage and thickness, replace them if needed