Engine wont start or stops
Lets just check that kill switch on the right handlebar before we go any further. Usually it will cut off all power to the engine so it wont even turn over. Other traps include having the sidestand down or not pulling the clutch in to start. Assuming all these check out, then you have two avenues to investigate:
To check for a spark, remove a spark plug, put the cap back on it, then ensure the metal collar touches the engine block to earth it. Ensure theres no fuel or vapour around (and never touch the plug or cap with the ignition on) then switch the ignition on and turn the engine over. If theres no spark, track the leads back to the coil, looking for cracks and secure connections. Also check the battery leads are tight.
Do make sure you havent run out of petrol. If theres plenty, check you havent turned off the fuel tap (where fitted). Next check for a blockage. With carburettors, wait for the engine and exhausts to cool right down then loosen the drain screw on the float bowl. If fuel flows, then its probably not a blockage. Check the throttle cable moves the slides inside the carb and the mechanism isnt detached.
If the float bowl is empty, work back over the fuel lines to the tank to find the blockage. It could even be a clogged filter inside the tank.
With fuel injection, roadside checking is pretty much limited to checking the throttle is actuating the levers on the throttle bodies. For this, and checking many carbs, youll need to remove the petrol tank.
Be careful not to spill petrol: pinch the line or close the end with tape or a suitable screw.
It is very rare for a four stroke engine to seize but it can result from working a new engine too hard or running low on oil.
On two strokes, seizures can be caused by overheating, the wrong fuel/oil ratio or a malfunctioning oil pump. A partial seizure may free up once the engine cools, otherwise you're walking home.
If you feel an engine is dragging and about to seize, whip in the clutch until youve stopped.