Thank you for your information. From what you have described, it does sound like due to time, wear, and stress on the electronics, the motor controller board has a component that has been damaged.
If you were still able to run it, and it shut down during use, I'd conclude that it is a wear issue amongst other things. But since you can not even get it to move, you definitely have a short.
A short is basically the cause of electricity going into the machine, stopping somewhere along the line (not making it throughout the whole machine), and coming back to the outlet.
The only place that this could happen, along with a "pop" that your wife heard seems to be at a damaged component in the controller board (near the motor). The electricity goes into the board, reaches the damaged component, bounces back into the outlet, and trips the breaker.
I'm sorry to say, but you will need to replace the controller board (item 72 of your owner's manual)to get it going again. Then you may have to lubricate your deck and belt (or possibly replace both) to prevent it from happening again. I'd be happy to help you with the procedure on that as well.
Depending on your warranty situation (which may not be applicable), the controller may be costly.
Here is the information for you to call or email them to find out what the cost would be.
Call: 08457 089 009
E-mail: [email protected]
Please let me know how it goes.