Unscrew and separate the barrel and and receiver. Clean and degrease the inside and outside threaded area. Make sure you can screw the parts back together. Separate the parts again and apply a liberal coating of red Loctite on the threads then screw the 2 pieces back together till the parts lien up properly then let the Loctite dry until tomorrow.
I've seen 2-3 Benelli guns and a few Brownings over the years that have come loose as your has done.