Simkarte ohne Funktion

Hi, I have exactly the same problem... I had prepaid SIM card an took over to contract plan. Now SIM card doesnt show any signal since date of transfer... Tried restart phone and to put SIM in another phone.. I was also in O2 shop and the guy told me that "everything is fine, it should work, call hotline"... I can't call hotline since SIM doesnt work. Please help...

Thanks,
Tom