I have a John Deere tractor mower that will not currently turn over. Thought I recently fixed the tractor when there was absolutely nothing’s happening when I return the key in the ignition. It turned out to be a blown fuse that I replaced. That appeared to fix the tractor completely. Along the way, I changed the battery, solenoid, and spark plug. Tractor was working fine when I was hauling some firewood, but as soon as I engaged the blades for the. Grass cutting, the machine turned right off. Currently, when I turn the key, all that happens is it sounds like the machine is trying to turn over, but cannot. Thank you very much and answer any help you might be able to give!

Fuses don't blow for no reason.  
 
It sounds like a wire to one of your safety switches, for blade engagement, is shorting out somewhere..
 
Start checking the harness for any shorts to ground.  
 
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Thanks for the info. I’ll check the harness and report back.

I may have found the issue. An electric lead that connect to the motor from the solenoid under the carburetor was loose. What ever was holding that in place is gone. I temp attached it and the tractor started right up.
Now I just need to find what type of bolt was attaching it in the first place. Nothing I found in my tin can full of nuts and bolts was working.

I took a similar sized bolt from the other side that wasn’t holding anything crucial on. I’ll still replace that one.
Tractor running normally now and mower deck working fine.
Thanks again!

Excellent!
 
Yep, that lead, that goes to the carb solenoid, provides 12 volts to open that solenoid.
With that possibly bare wire flopping about, first it'd short out and blow a fuse.
Second, the carb solenoid wouldn't "open" to allow fuel past the main jet..  It'd
literally run out of fuel with a full tank...
 
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Thanks again for all the info. I’m definitely a beginner with this stuff but I learned a lot before finally finding that loose electric lead.