Smartphone becomes "Off line"

The word "Off Line" suddenly appeared in my smartphone Nokia X6.

I have taken out the sim card & inserted it again, but it still not work.

Please advise what is the problem & how to turn it to normal.

The sim card was OK when I used it with other phone.

Before it was out of order, I have gone into the meno & pressed some items, but
forgot which one.

Is this is the case which turns the phone to "Off Line" by pressing mistake.