My phone number is being used by someone elses

i have been using my cellphone number for the past 12 years and recently travelled abroad for about 4 months. I was on roaming until about 3 weeks ago when i missed my flight and suddenly i stopped receiving even sms when i inserted my sim card back into the phone after just an hour. When I got back to the country yesterday i realised my number is ringing and someone else is using it. I really want this issue solved as this is my only phone number and all my banking and business details are linked to this phone number.