The printer was probably installed to a certain IP address. When you turn it off and on again it probably gets reassigned a new IP address by the router. (if you have DCHP on).
I don't think it's a good idea to turn off a network printer, we have two and keep them on all the time. If we turn off one of them it never remembers what IP address it used to have (the other does though)so I have to reinstall it!