When I try to make a call with my Samsung Galaxy S4 the mobile network not available message appears. How can I fix this error and make a calls again with my Galaxy S4? I already tried to restart my phone, and call again, but again got a mobile network not available error.

I tried to select manual network from settings, but the message mobile network not unavailable was appeared again.
I also tried to re-insert my sim card, but still can't make calls. Yesterday my phone was fine, but today I can't make calls. How to resolve this error? Please help me.
Michael