Could be a number of things that cause the mower to stop self-propelling.
Belt has come off
Belt has broken
Cable has become disconnected
Cable has broken
Debris/clippings, etc. are binding the rear wheels or belt or input pulley on the transmission
The HRR2168VKA came with a 36 month warranty. Note the warranty covered factory defects, so if the problem is NOT due to a factory defect, it is not covered under warranty. Any Honda dealer can evaluate and inspect your mower. If you go that route, be sure to fully communicate with the dealer what is and what is not covered, before any work is done. You can find a local Honda dealer using this link:
Find A Honda Dealer
When you call, have the full serial number off the back of the mower deck; this will help the dealer look-up your effective warranty start date, as well as get correct repair parts as necessary.