Motor won't start only clicks and can not move flywheel by hand

It could be many things. Not all are terrible.

1) Try removing the spark plug and seeing if you can move it bay hand then. If the carb has leaked gas into the engine the cylinder will get full of gas and you will not be able to turn it. When you turn it by hand gas will come out of the cylinder if this is the problem. If gas comes out you need to change the oil and get the carb cleaned out.

2) If there is not gas in the cylinder but removing the plug makes it easier to turn then you probably need to adjust the valves.

3) If neither of these make it easier to turn the engine remove the cover over the starter and see if the gears are stuck engaged to the flywheel. If the starter is stuck engaged it will not want to crank.

4) If none of these make it easier to turn the engine then you probably have some internal engine problems.