Hi Angel, I'm sorry to hear you're having problems with your camera!
The first thing that you want to do is to actually confirm that the shutter is stuck open since you don't seem to be totally sure. Take the lens off and see what is going on. You won't mess anything up by doing so as the frame of film that is behind the lens right now has already been overexposed if the shutter has been left open. Is the mirror flipped up but shutter closed or is the mirror up and the shutter is open (you can see the film when you remove the lens)? If the film is still being exposed to light, eventually the image you took before this happened is going to start getting fogged. So if you don't find a way to close the shutter soon, you should rewind the film. If the mirror is stuck up but the shutter is closed, your film is safe and you can proceed to reading the instruction manual to figure out how to get the mirror down.
If the shutter is open and you are at the middle of the roll and don't want to waste any more of it by just rewinding it all back into the canister, you can simply make note of the frame number you're on then rewind the camera slowly until you hear/feel the first 'click' of the leader coming off of the take-up spool then STOP REWINDING before you hear/feel the second 'click' of the film leader being sucked into the canister. Then you can just insert the film into this or another working camera later and shoot with the lens cap on until you get 2 frames passed where you left off.
Now that your film is safe, or if you're in subdued light and want to try with the film still in the camera (or if the shutter is actually closed) work the advance lever while pressing the shutter release in succession several times. Basically, try to 'wiggle' things till the come unlocked. Believe it or not, sometimes this will at least get things working again temporarily.
If that doesn't help, remove the battery. With some electronically timed shutters, simply removing the battery will close the shutter when it gets stuck. This can indicate that the battery is bad. Even if the shutter does not shut at this point, if the battery is drained, installing a fresh battery could resolve the problem.
If none of this works, review the users manual and be sure that you are operating the camera correctly and nothing is engaged that shouldn't be. Here is a link to it:
http://www.butkus.org/chinon/canon/canon_at-1/canon_at-1.htm
At this point, I would say that the camera is broken and needs professional help which will usually cost from $100-$150. I recommend CameraRepair.com for that and KEH.com for a "new" 35mm SLR such as a Canon FTb if you'd prefer to go that route. The FTb is tougher and can take the same lens you use on your AT-1.
In the meantime, get your film out of that camera and borrow one from your teacher or other student so you can get your assignment done!
Good luck!
METAL AND MANUAL! FILM FOREVER!