Recently, I started hearing this high pitch squeaking sound coming from my front rotors when I come to a stop. Also, my rear disc seems heavily damaged because I see visually grooves and feel big ridges. The rear brakes simply stopped working and I haven't ridden it since. What's my next step? Do I need a new rotor in the rear?

 

new rotor - yes.

More important, find out why there are grooves in the rotor. Most likely, the pads are completely worn out and you've gotten down to the rivets or backing plates.

Which - that in itself is weird, as sportbike riders rarely ever use the rear brake.

The fronts could be the same thing - the pads are worn completely down and you might be rubbing backing plate on the rotor, or there might be some trash embedded in the pad materials.

Since on a motorcycle, your brakes are your life I'd immediately pull all 3 calipers, inspect all the pads and find out what the problem is.

 

Pulled the calipers off this weekend. But to my surprise, the front pads were fine. I guess it's normal that it makes noise? The rear brake pad was close to new! Found out that my rear brake lever was bent badly and had lots of play from being dropped. I have to get around fixing that now...Thanks for the help!