The bank denied my fraud claim. why? what now?

I am a freshman in college. I am 18. In the beginning of the school year I opened my first bank account with TCF. I wrote my pin # on a piece of paper and I kept it in the same place that i kept the card and i also kept some checks in my wallet. I didn't know that it wasn't a good idea to write the pin# down. about a month ago, I lost the whole wallet, but i thought i had lost it in my house somewhere and i am away in a different city at school. with the type of account i have i can use my student i.d. as a debit card so i just took the little money that i had on there off and thought that everything would be fine. I was sure that the wallet would turn up. I waited a couple of days to search for it and have my grandmother at home search for it. I couldn't find it so i called the bank and reported it lost. I found out that someone had deposited a bad $3000 check in my account and tried to take out the money. They managed to spend a little over $1000 before they account froze. when i lost the card my only worry was someone spending my money, so i took it out. but i had no idea that someone could do that in just a couple of days. i filled a fraud claim of unauthorized use with a manager at a branch. they denied my claim saying that no error occurred and that i had some gain out of at least one of the transactions. THIS IS NOT TRUE. i did not even know that my card was being used. The manager at the bank said that he would call them for me and ask for more info. but right now i owe the bank $968, because they took a work check that went into the account before i could stop it. Why should i have to pay for transactions that i did not do??? I do not have $968. I do not want my credit to be destroyed because i lost my card. what should i do?