You can't fix it without access to the files and the db so don't give out your account details.
Edit: actually do you have cpanel access or are you on a free forum host? If you are not on a free forum host then your host can't do must because if you have cpanel and web hosting and installed phpBB yourself then your host won't help you.
Your host should keep backups (you should make backups aswell, everyday.so this wouldn't happen again and if it did it wouldn't be a big loss as you have a backup in the last 24 hours.) ask your host for a backup losest to the date before you couldn't access your acp.
Change your password for your forum and cpanel and don't give it to anyone