Voice, like all statuses, are temporary. Once you've left the channel or disconnected from the server they are lost. To get voice again an op will have to voice you again.
What the channel owner might have meant is that you have been given some sort of automatic "AutoVoice" status which isn't so much an IRC server status as it is 3rd party services or scripts giving your voice back automatically.
If you're autovoiced in ChanServ you may need to IDENTIFY to your nickname before joining the channel. This is usually done through /identify password or /msg ChanServ identify password or just /chanserv identify password! Try that.
Otherwise, you'll just have to ask an op to let you have voice back. Maybe he didn't mean you to have voice automatically when you joined every time and it was a one-off thing?
When I asked the channel owner what had happened he asked if I'd used my password. That's when I started looking at the chanserv stuff for what to do.
My nickname is registered and therefore passworded, so do I use my nickname password on those chanserv commands you gave?