When a blade on a push lawn mower becomes locked in place and not movable, something is either stopping it from turning or the engine has seized up.
Remove the spark plug boot from the spark plug and then tilt the push mower backwards towards the handles. Look under the mower deck to see if a stick or rock has been caught up between the blade and the deck. Also check to see if the deck has been damaged and the blade is hitting it and stopping it from turning.
The turning lawn mower blade can also be bound by wire, a dog cable or an electrical cable as well as long grass or vines. Make sure that these areas are clear at the crankshaft which is normally where they get wrapped up.
If the deck and crankshaft areas are clear, then it could be an internal problem. You can try to slowly move the lawn mower blade to see if you can free up the crankshaft. If it does move, then check the oil in it to make sure that you did not run it low on oil. You might be able to add some oil and start the engine but more than likely, the damage has already been done.