You can turn it both ways to see how sharp it is.
10w-50
If you have a belt driven blade set up either the belt has jammed the pulley because it is worn out or the belt is broken. It could also be that there is a problem with the pulley's, but this is rare. I agree that the belt is the most likely problem. But there may also be a problem with the cable or linkage that engages the belt. If your belt looks good then have someone try to engage them with the mower not running and see if everything looks like it is moving properly to stretch the belt and engage it. If this is a push mower then the spline on the blade or crankshaft is probably stripped.
A Razor, a lawn mower, and a rollerblade all include a blade. A razor has a blade. A lawnmower has a blade A rollerblade has no actual blade but it has the word blade in its name.
if its an electric clutch the solonid that engages it may be bad, if its a manual engagment,the belt may be bad,or a spring has come off,
How do you change the blade on a Toro 21" self-propelled mower
Usually a short length of 2x4 can be used to wedge and hold the blade to loosen the bolt.
The Yardman Series push mower uses a 21 inch blade.
What is better a high lift lawn mower blade or a low lift blade.
Most lawn mowers have safety features that will turn the engine off when there is damage to the blade assembly. That is very likely the problem.
The blade
If your push lawn mower is not working as well as it used to work, you might be thinking about replacing it. Before you purchase a new push lawn mower, check your current mower's blade. Over time, lawn mower blades grow dull and no longer cut grass as effectively. If you take your lawn mower blade to a hardware store that offers a blade sharpening service, you can usually get the blade resharpened for a small fee. With a newly sharpened blade, your old lawn mower could be working just like new.