Mower blades willnot engage

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

check reverse safty switch or engine rpm to low to run pto

look at the fuses there maybe a blown one

check the fuse if ok check the safety switches

check for power at the clutch if no power check the safety switches and the switch itself

You have several safety switches on your unit. The engine should not start if the PTO safety interlock is engaged. Be sure PTO switch is off.
If the sarter Does Not Work Or Will Not Turn Engine Brake pedal is not depressed.
or the PTO is engaged.
Battery terminals are corroded. or Fusible Link in starting circuit is blown - (Not likely unless bad wires) Here is a link to your wiring diagram (maybe your model)

http://www.postimage.org/image.php?v=Pq1tOfiS
Notice how the starting circuit goes through the brake pedal (must be depressed) and the PTO switch (must be disengaged) and that the PTO only works with operator present. Print that page while link still works.

Park brake switch is the most likely culprit,any other would cause the tractor to not turn over in the crank position.

sounds like the seat switch is bad

have a john deere zero turn that will not turn all the way over