Do you really have to go to the dealership to fix this problem? Or can anyone give me some advice?

 

More than likely the radio still works but you just need to reset all the stations. When the battery was removed the radio had no power to keep the channels stored.
Check your owners manual for presetting stations.
If radio is truly dead, check the fuse for the radio.
Might have had a power surge in system.

 

A lot of cars have anti theft on radios so if they are removed they wont work same if you remove battery, usually have to punch code into radio to get it to work again.

I know Mercedes Benz does that, not sure about MINI