Kohler diesel generator won’t start.

Ah, the ole 3TNE. Darn good Yanmar engine.
I'm in Satsuma, just to your Sou-West.

Lets talk a lil more about suddenly stopped running.
Was just tooling along making power for several minutes/hours and stopped running like it was turned off?
Turned off with sputters?
Slowed down do nothing?
Made any sound or smoke?
Nobody adjusting supply or return valves?
How much fuel is in the tank it is drawing from?
You have dumped the secondary fuel filter and was clean fuel? No water? And you re-filled a new filter with fresh fuel?

Don't mean to over whelm you with a bunch of silly questions but we need to get on the same page as you and ensure the lil things are understood.

Are you mechanically inclined?

Rite off the bat, there is a fuse, relay and known flaky plug connectors.
The injector run enable coil could be an issue also.

That's just on the outside of the engine and the list of usual suspects.
But wait, there are more worms in that can but we need a hammer and wrenches to check the next list.

Well, lots of the stuff offers protection for the engine in case of low oil or no raw/cooling water.
Other bits of stuff offers easier starting and shutting down.
Despite this stuff back firing and inducing errors sometimes, it is still better with, than without.
Then, I think it was the small Kohlers that also checked that the electrical end was indeed making 115/230Vac or would shut down.
Oh-No offered over/under speed protection.
Aluminum exhaust manifolds were my pet-peve.
Then error codes, displays, and network interfacing.
Meant to look and offer hy-tech options, but just hy-tech headaches were realized.

I see Capt Denny has responded..

Have your tech ensure a run voltage is getting to the run solenoid. Mounted on the rear of the injector pump.
While it is connected, get a pointy probe into the in-line connector and ensure 12Vdc is present.
If so, I suspect a bad run solenoid.
If 12Vdc not found, An error condition (real or failed) is keeping it off for some reason.

Also, your model may have hidden in-line fuses inside of the control box. Pain to check but is a must to look for if nothing else pans out.
Also #2. That D%$^ Kohler relay board luves to bite techs in the asp. Yes, they do go bad.