I have created an account in testnet which I know the address and the password of, but I did not backup the private key file. Can I somehow create the key file again using the password? Or is my account locked inside forever?
You cannot recreate the private key file. The password is used to decrypt the key file, and then an address is generated from the private key in a non-reversible way. The private key is really the most important piece and should be backed up.