Welcome to Allpar. This is protected information, as it should be for an effective anti-theft program. You would have to prove who you say you are as well as that you own the vehicle indicated.
Please understand that I would help online if I could.
There may be a credit card-sized code card in the glovebox or underseat drawer with a code to be entered into the Alpine radio in order to unlock it.
The Chrysler dealer would be able to help once vehicle ownership is proved. Keep the code in a safe place after this in case it ever occurs again. There could be a dealer service charge if it has to be written up on a work order or it could be a courtesy look-up at the parts counter. Bring your ID to the counter with you.
I see online resources for radio codes, but you have to leave an e-mail address and it isn't free of charge and I can't vouch for the integrity of this service.
Best of Luck to you.
For the future, you'll need the code any time the radio loses power. The radio has no way of telling whether it's just lost power or if it's been stolen. If you have the code, you should be able to enter it into the radio somehow, and and you can avoid the trip to the dealer. If you have to get the radio unlocked by the dealer, have them give you the code, and make sure you know how to enter it, since you may well find yourself in this situation again if your battery goes flat or has to be removed again.