There are many reasons why a nitro RC engine might get stuck or completely seize up altogether. In no particular order, here are some common causes of a stuck nitro engine:
Too much fuel in the engine / combustion chamber (Hydrolock)
If there is too much fuel in your combustion chamber it can cause ‘hydrolock’. This can happen if you have repeatedly been trying to start your nitro engine and it has flooded to the point where it feels jammed. To fix this, undo the glow plug so it is loose, and then see if you can turn the engine over a few times. If it does, then keep turning it over until you have cleared the excess fuel out.
Stone or debris jammed against the flywheel
This is a common problem if you run off road a lot. Check for small rocks or stones that might be jammed in between the chassis and flywheel and carefully remove them to free up the engine. It sounds silly, but I have seen it several times.
Pull-start cord Jammed
If the pull-start has become jammed this can prevent the engine from being turned over. You’ll probably need to remove the pull-start and check the engine for free movement. If the engine is fine then the pull-start probably needs rebuilding or reassembling.
Has the engine been sitting for a very long time
If your nitro engine has been sitting for a long time, any old fuel can become sticky and clog up in the engine. In extreme cases the engine may feel seized or difficult to turn over. Try pouring a few drops of after run oil into the piston and turning it over a few times. You’ll need to remove the glow plug and pour a few drops in the hole, then with the glow plug still removed, try to turn it over a few times.
It is good practice to use after-run oil after each time you finish using your car, and very important if the car is going to sit unused for some time.