Fixing your problem will depend on the details. If you are saying that you deleted a FB App from a phone or tablet, say, then you'll just need to go the the apps store and download it again - it's free on pretty much any platform. You'll likely have to log back in again once the app is re-installed.
If you're using FB on a computer browser, they you should be fine to just go back there and log in again.
If you're saying you actually deleted / cancelled your actual FB account., it should be very difficult to do that accidentally. From FB posts such as this
FB goes over the difference between deactivating and deleting an account, and pretty much makes you explicitly petition them to delete the account.
If you didn't do that, you account is likely just fine. Check it with a PC, and, I'd imagine, start with sending FB a forgot-my-password request from the FB login, which will send a reset link to your e-mail. That, or FB will say 'no account associated with that e-mail.' If that's the case, you're pretty much out of luck.
Hoping that helped!