Sigh. Doesn't anyone know you can use a custom recovery to attempt to retrieve data out of the phone? You need someone like TWRP (Team Win Recovery Project), Clockworkmod or Philz touch recovery.
I know with this method you can retrieve data from the phone while in recovery menu. If I can remember you need to mount system and data in recovery.
Otherwise just flash correct firmware from Sammobile (make sure correct model number, region (service provider or country)) before downloading the firmware and use the program called ODIN which instructions are given on the sammobile website when downloading firmware.
When you do flash it via ODIN it will not wipe the phone. It will simply either attempt to fix the system firmware or update the phone. Hopefully this is enough to break the boot loop and get it to working.
The extreme method which is expensive data recovery is taking it to someone that is specialized in doing so.