Troy Bilt Pressure Washer backfired and now it won't start

My dad was using his pressure washer the other day. When he shut it off, it backfired (super loud) and now it will turn over, but it won't start. It is a Troy Bilt, Model # 020414, 2700 psi with a Briggs & Stratton engine.  It's called a "no prime / no choke Ready Start" 7.75 foot lbs of torque.  He said he put gas in the head, that did not help.  He checked the plug and it's clean.  He also checked for gas in the oil (there is none.) Any thoughts?  Thanks in advance!