Charging for non solved problems ?

What does anyone do when you go to a site for a quick visit and find the problem is going to take over expected time to solve?
Eg I had a client who lost all his data on an NT workstation (no backup)and decided to re-install NT himself. He did nottell me this on the phone, just that he had a problem seeing the network on NT Server (even though he did not have NT server. When I got there he had a completely differnt problem than he described…and it required a reinstall of NT ..hence upto a day to get the network back to scratch.
I told him thiss and that I did not have time to do it immideatly due to other client committments. So should I cahrge for the visit or not?