Will it return to its normal shape once the boat is off the trailer or is the hull ruined?
Why no just take it off the trailer and try it?
You will get a 100% accurate answer by doing so, instead a bunch of people who can't physically inspect the boat from making guesses.
After all without knowing how long the boat has been like that (you don't even know) , if there are signs of cracking etc.. We would have no idea... And either way it would still be a guess.
Having seen fiberglass boats abandoned on trailers for years, with their cockpits full of snow and water, I would guess you are OK.
But why guess? Inspect the boat for holes so it won't sink.. And launch the boat.