Hmmm. Have the same problem after 4 yrs of perfect operation. We were out in very cold weather so I suspect a freeze problem.
Removed the existing backflow preventer and it was clearly not working or allowing flow in any direction.
Tested first then installed a new brass backflow preventer and no luck!
Heading out for a week tomorrow so will try the soapy water soak method to see if that unclogs anything.