If you crashed while flying, you might have broken some of the quadcopter’s arms. There’s no easy way to fix this, so just disassemble the frame and replace the arm with a spare.

After crashing, check if the battery is damaged. A damaged lithium polymer battery can explode! Immediately after the crash, disconnect the battery immediately if it has not exploded already, and inspect it before judging whether or not you can keep using it. If it’s dented, bulging, bent, punctured, or cut, then dispose of it properly.

Broken propellers are easy to replace, but make sure that the shaft or motor isn’t damaged. Remove the propeller first, then run the motor at a low speed to see if it’s still functioning and not wobbly. If it’s wobbly, then you should replace the motor, do this while not flying, or else you risk having the motor fail in mid-air which is more dangerous.