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Never had a problem starting my John Deere ZTrack F620 until two days ago when all I hear is a loud thump when I turn the key. I charged the battery and even tried to start it with my battery charge engine start. Its almost as it the engine is frozen. Can it be the starter? The starter seems to engage, but it won't turn the engine. There was no fore warning of this event. I checked the oil and its clean. What can it be?

pull spark plugs, inspect plugs & check gap per manual, rotate top of engine screen by hand & reinstall plugs & give it a go.

How do you rotate the top of engine screen?

mechanic mark said:

pull spark plugs, inspect plugs & check gap per manual, rotate top of engine screen by hand & reinstall plugs & give it a go.

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by hand

skiperkie said:

How do you rotate the top of engine screen?

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Mark is not aware that this is a top quality machine with a horizontal shaft V-Twin engine.
To turn the engine over by hand is a bit more complicated
You will need to raise the hood then remove the front belt guard ( nearest the driver )
You will then have access to the top PTO shaft and the pump motors.
Turn any one of the pulleys but try to avoid yanking on the belt.

You can not use the rear PTO shaft because it has an electric clutch on it.

Because it is an older mower and a full professional one it had an engine 10 times better than any vertical shaft engine.
Down side is it can be a lot harder to work on it.

The loud thump made Mark consider a broken con rod which is why he suggested turning over by hand.
In many cases if a rod lets go when starting, you do very little damage internally so the engine is repairable.
However repeatidly cranking with the starer motor can cause a broken rod to smash through the side of the cases after which the engine is scrap metal.

So check both pistons are going up & down is the first step.
The second one is to remove both rocker covers to confirm that both sets of valves are going up & down.

If the engine will not rotate you will have to pull it out to do any further work on it.

I can not recommend strongly enough buying the JD technical manual for your mower .
They are written so the average 10 year old could pull the mower down & reassemble it.

Is it possible that the PTO switch may have gone bad on the ON position and therefore, when I turn the key, all I hear is a thump?

Yes.

Thank you very much. I will check it out. I have already replaced the spark plugs with the correct gap.

As I've always been taught, start with the inexpensive fixes first. Ignition switch, seat safety switch, PTO switch, etc. Continuity checks along the ignition circuit sounds in order.

If you hear a bang, or thud and he engine won't turn, the first thing is to look for the cause.
And you look for the cause in a manner that you are not liable to do any more damage.
This is why Mark told you to try & turn the engine over by hand, which I will also concur
There are only a few things that will stop the engine rotating and all of them are made substantially worse by continuing to try & make the engine start.
You can start replacing cheap things like plugs but it will make no difference if the stator is locked by a rock jambed between it and a magnet or a con cod is broken or a pushrod is being wrapped around the camshaft.

If it does not turn over easily by hand it will never start.
However it is your mower & your money so if you want to keep wasting money replacing perfectly good parts in the hope of apeasing the mower gods so they will breathe new life into your mower then go ahead.
You do not need any help from professional mower technicians to do that.

I did check the PTO switch. The connections were corroded. I cleaned them and it still would not start. In desperation I banged on the PTO slightly. To my surprise, the Z-TRAK turned over. Now it seems like the carburetor is not getting gas. No fuel coming from the fuel pump.