Go on the vodaphone website and see if you can send them an email. I had the same message from Virgin once. I got a mobile in case my car broke down, and I put the phone in the glovebox, and left it there. I didn't realise that I had to use the phone now and then to keep the service 'active'. Now, I use it once a month, just to keep it working.
It means that your phone isn't getting a signal from your network provider.
It could be a problem with the network itself, or the problem could be at your end, insofar as you've been disconnected, or you're in a place with a poor reception ......