Well there are many different things that can go wrong with and engine and make them bad or "fried" as you put it. A few vague examples are:
-If the engine is seized up it will not start.
-If it does start up but knocking then it needs to be taken to a mechanic. Driving a car that knocks will only cause further damage.
-A car with a blown head gasket will start and run usually but will overheat and cause further damage such as warping of the heads.
If a car doesn't start it doesn't necessarily mean that the engine is "fried". There are many possibilities for why an engine will not start. Look up "no start" situations for the car make and model. Hope this helps.
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