When rebooting, why is my Samsung A3 downloading for such a long time?

[AC Question]

It says Downloading... Do not turn off target. What is going on?

 

[Rukbat]

You booted into the download partition. Try pressing the up volume control. If that brings you back to the downloading screen, your main Android partition is gone, and you'll have to flash the ROM. See [Samsung] How to flash Stock ROM via ODIN. (Yes, you'll lose everything - but it's already lost if the phone won't boot into Android.)