Kate, the serial number is on a sticker on the rear of the mower deck; looks like this:
Honda recently issued a Service Bulletin that may help; the issue is
on some mowers, including the HRR2168VKA, the automatic choke (crucial to start the engine) may be binding up, making starting nearly impossible. Honda has improved the design of the choke and your mower may benefit from this new part. If a Honda dealer confirms this is the problem, and your mower is less than 3 years old, it will likely be covered under warranty. I can confirm how much warranty you have left if you'll share that serial number with me.
When taking your mower in for service, be sure to clearly communicate with the service manager if/when you will approve for any work to be done ahead of time, just in case the problem is NOT covered under warranty (for example, stale gasoline in the fuel tank)
Here's the link to find a Honda dealer in your area that can offer service; just select
Lawn Mowers, check off
Service and enter your
ZIP code for a list:
Find A Honda Dealer