Nokia N73 offline mode problem

Hello,

I have a Nokia N73 cell phone that when I switched it on, it suddenly became Offline mode. There is no signal and when I put it on General profile and press to activate it, there is a message "insert sim card" although the sim card is already inserted. Also many forums says that when you switch off and switch on, the phone will ask whether you want to continue in OFFLINE MODE, select NO and it will be okay but there is no message like that. By the way, when I use my sim card to other phone, it works perfectly (and I put also other sim cards to my N73 but still it is on offline mode). They said that this Offline mode is perennial problem of N73. Please help me find solution for this problem as I am not able to use my cell phone anymore. I will appreciate any help and/or suggestion you can share. Thank you very much.