Encrypted external drive no longer asks for password

I have an external drive that I encrypted and password protected a while back. I noticed recently that it no longer asks for a password when connected, which defeats a lot of the purpose of this security. It appears the drive is still encrypted, but I don't know if there's a way to restore the password protection without formatting the drive and starting over, which I really don't want to do. Also just seems like a major security flaw for the password prompt to just stop one day. Any way to fix this? I'm on El Capitan.