Can you rotate the blade by hand? Obviously make sure that the power is OFF, either by removing the plug from the socket if it's an electric model or by removing the spark plug lead if it's petrol powered. Then try to rotate the blade. It's possible that it is clogged up with compacted wet grass. If this is the case, scrape it clean with an old screwdriver or similar implement then give it another spin by hand before you turn it the right way up and power it up again.