What ingages blades ,blades not engage - Garden

Generally when I have that problem it is to do with worn or damaged blades. I have had string, rope or something on blades which make them unbalanced.

https://www.searspartsdirect.com/riding-mower-tractor-repair/symptom/LTRTR140.html
https://www.lawnsite.com/threads/why-my-deck-shakes-when-i-engage-blades.241006/
One below was a problem with belt
https://www.doityourself.com/forum/outdoor-gasoline-electric-powered-equipment-small-engines/583989-lawn-mower-deck-vibrates-sounds-real-bad-when-you-engage-blades.html

Check the breaker switches 1 s under the brake one is under the seat and some have one under the blade engagement lever.

Here's the link and I'm sure you will get the solution of your problem.
JOHN DEERE D140 SERVICE REPAIR MANUAL

Check your blade ingage switch. Sometimes they stick. You can jump start it by jumping across the solenoid switch beside the engine. I sometimes have to engage and disengage my blade switch and it starts. Hope this helps.

look and see if the belt came off the pulleys, it could be at the front pulley or at the rear pulleys. lower the deck all the way down and look.

I would check the belt, the pulleys and the tension of the pulleys. Any of these could be the cause of your issue. Hope this helps!

After hitting a tree root, blades will not cut when I press to engage

just replaced some pulleys on my craftsman 42 inch cut and now the blades are ingages all the time, but it will start

check your Reverse Gear engage switch and PTO (blades engaged switch), either a bad switch(es) or short in wiring indicating that you are in Reverse with the blades engaged. Is there an override function so that you can mow in reverse? Check that circuit vs. the PTO and reverse gear. good luck

is it electronic engage or manually?