Contact your network provider (o2, Orange, EE for example) and they will be able to reactivate your phone with the existing number you were using. This process doesn't take long infact I had to do it myself less than a week ago.
The other alternative with the same end result is to just go into a mobile phone shop and ask them to activate your phone with the existing number - this will need to be a shop that deals with your network provider (EE tend to deal with most networks but I know others do too).
Answers & Comments
Contact your network provider (o2, Orange, EE for example) and they will be able to reactivate your phone with the existing number you were using. This process doesn't take long infact I had to do it myself less than a week ago.
The other alternative with the same end result is to just go into a mobile phone shop and ask them to activate your phone with the existing number - this will need to be a shop that deals with your network provider (EE tend to deal with most networks but I know others do too).