You have to provide much more info liek what you actualyl used in your project, what system and os on. and yeah, auto-save may sometimes save the day, but this is even better:
Thank you for your help! I actually just found a non-corrupted file of my project on my external hard-drive's back up, luckily. I'm not sure how I can prevent this from happening again though.
But still, thank you!
Oh and I'm on Windows 8, and I used the plug-ins that come with AE, and optical flares. But I highly doubt that's the case. Last night, I forgot to shut down my computer, and the next day I got an error saying 'Your PC ran into a problem and needs to restart' and after all that, I opened AE and I got another error (I can't remember what it said though something about AE.) So after all that, I opened my project and got the error mentioned in the question. I believe my file got corrupted during the random restart of my PC. I guess I should've stated all this info before, but I was so worried I couldn't think straight. But since I found another saved version of my project in my back up, I'm fine.
Oh and thank you so much!