I have a 68 riding tractor. It has an 8hp briggs & Straton. It started fine, I backed it off the trailer and it died and refused to start. I replace the plug and it started, it ran for a short period, died and now I have no spark

Pull the plug and see if it is wet, Carb?
Was thinking you may have severed a wire backing up, but you say it started since then.
How does the plug look?

Plug looked fine, put in new one. Grounded plug and turned motor over, no juice.
Thanks

Sounds like the coil went bad.