Sounds like a regular power mower, not a rider mower that you are talking about. If the starter won't pull, can be a jammed recoil clutch or jammed starter rope, but most of the time either grass cuttings or twig opr a toy or such jammed in underneath, or the sheet metal has been bent hitting a rock or because it was rusted out and is interfering with the blade.
Paying attention to turn it on its side so the fuel tank is down (so you don't dribble fuel over the engine), first disconnect the plug wire and jam it against metal so it is well grounded - usually into the slots of the head fins works well. Remember never to get your hands into the blade area when mowing the blade (which can make it start) - use a stick or string or piece of rope or such to turn the blade around, looking for blockages.
If the grass was damp, it can wad up around the inside of the housing and block the blade - if real long or tough variety sometimes wrap around the shaft above the blade (talking rotary mower here) or at ends of blade (on reel type) and jam so tight that the friction on the shaft prevents it from turning - cut away with a knife or unravel. Pieces of string or wire or plastic bag can also do the same thing - wrapping so tight on the shaft that it cannot turn.
Also look, while rotating the blade, for sticks, piece of metal, kid or dog toy, or bent mower housing prevent the blade from turning.
Otherwise - assuming you made sure it is not out of fuel or the choke did not accidentally get turned on, and that the plug wire is securely on the plug, could be electrical or carb or internal mechanical problem - for those sort of things probably beyond your repair abilitiy if you came here for ideas. Oops - you said it would not turn - so mechanical in the engine or jammed recoil clutch or sheave or cord would be my first guess if the bladeis not jammed. I can think of about 10-15 things which could fail or break, and about 5-6 of them could prevent it from turning.
