Without the make and model there is no way to tell what the problem is.
There are four different ways for blades to engage. They can engage by cable, linkage, belt and electric clutch. If your mower has cable or linkage you will need to have someone operate the lever and you will need to watch under the mower to see what is moving and what is not.
If it is driven by moving the deck until the belt tightens you will need to make sure that the deck is moving far enough forward to loosen the belt and that the deck pulleys hit the little brake pads that stop it from turning.
If it is cable or linkage operated you will need to see where it moves the linkage to tighten the belt and see what it not operating correctly.
If it is electric then make sure that you can turn the clutch when it is not engaged. With the mower off of course.
Most of these options still need the pulleys on top of the deck to be stopped by an arm with a brake pads on it when the deck is disengaged.
If none of these answers help then please ask again with the make and model and we can try to get more specific answers
There could be many reasons why your Sears riding lawn mower won' shut off. The key could be jammed for example.
your electric motor(I assume) probably wont have enough torque, That is on a Riding type mower, not a push mower, it might work on a push mower as long as its not self prepelled, because your motor probably still wont have enough torque!
check the front and back belt pulleys and if the elt is on the right side of the engager.
You will have to put the rider on a ramp in order to get under it to replace the blades. For lack of a ramp, I leaned two 1 x 12 boards from the ground to the edge of my back porch and pushed the rider up so the front wheels would be on the porch. Had to lie down on the ground to do the work, but defintely worked ok. Regardless of which method you use, be sure and block the wheels for safety,
Perhaps it is the starter drive. That happened to my riding mower. Got the drive replaced, works fine.
Your flywheel key might be sheared
Pins on the brake disk on transaxle are rusty and stuck.
Mine is doing the same. It is still under warranty, but they will not honor the warranty, so I do not know what else to do.
torque flight transmission
replace battery & starter solenoid.. possibly carb.
clean the air filter (after each use!), check spark plugs, look for anything wound around the blades like a string or wire stoping it from turning.
(1)The battery may need charging, or (2) the battery cable(s) may be loose or need cleaning, or (3) the starter relay may need to be replaced.