My craftsman lawn tractor 42" cut (2007 model) cuts off when i try to engage the cutting blades

Your lawn tractor will have a lever that raises and lowers the mower deck which houses the two cutting blades. By lowering your mower deck to the desired cutting height, the blades should automatically engage. By raising the deck back to it's highest position, the blades will automatically disengage. Should be simple enough. You can consult your mower manual. If you don't have one, you can easily find it online using the make, model and year of the lawn tractor.

check the safety switch on the seat

Went out to cut grass and the blades will not engage and when I put it in gear it does not move. Belts are intact. I cleaned the deck of dirt and grass. Still nothing. Just replaced the cable to engage the blades last year. Cable seems to be working.

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

Safety switch at seat or reverse. Mower will cut of when blades are engaged if you get off the seat or put into reverse. Check these switches for stuck on, damaged, disconnected, or just plain broke. Good luck.

Unplug the engagement clutch[it bolts to the motors crank] and try to engage again.If the fuse does'nt blow you need a clutch.If it does still blow you need a switch or you have a short in the harness.

Is the belt on the pulleys ? If so your cable is broke its what engages deck belt

Check the blades to make sure that they are not upside down

It sounds to me like a very loose belt or very worn out belt and also an adjustment problem.

It sounds to me that you need a new mower deck drive belt, or your blades need sharpening. If the blade edges are rounded, they won't cut butter... Also, when mower is off, check mower deck drive pulleys to make sure you don't have any that are locked up or are dragging. You can check this by pulling the belt away from pulley and spinning the pulley by hand. MOWER MUST BE OFF before attempting any of these checks. You would be surprised how many people would try this with the mower running.
Hope this helps