Bad fuel mix – A bad fuel mix can happen in several ways below are some listed:
Your engine is out of fuel, so the engine takes much air but no fuel.
Your air intake might be clogged, so there is sufficient fuel but not enough air.
The fuel system might be giving too much or too little fuel to the mix with, meaning that combustion does not happen properly.
There might be some dirt in the fuel (i.e water in your gas tank) that prevents the fuel not burn.
Lack of compression: If the charge of air and fuel is not compressed properly, the combustion process will not work like it would. Lack of compression might happen for these under listed reasons:
Your piston rings are bad (letting air/fuel to leak past the piston during compression).
The intake or exhaust valves are not closing properly, again allowing a leak during compression.
A hole in the cylinder.
The most regular “hole” in a cylinder happens where the top of the cylinder (that holds the valves and spark plug that is also known as the cylinder head) attaches to the cylinder itself. Normally, the cylinder and the cylinder head close together with a thin gasket pressed between them to enable a good seal. If the gasket breaks, small holes occur between the cylinder and the cylinder head, and this hole creates leaks.
Lack of spark: There may be no spark or weak spark for a number of reasons:
If your engines spark plug or the wire leading to it is bad, the spark will be weak.
If the wire is broken or missing, or if the system that sends a spark down the wire is malfunctioning, there will be no spark.
If the spark happens either too early or too late in the cycle (if the ignition timing is off), the fuel will not ignite at the right time, and this can cause all kinds of problems.
Other things that can cause dead engine include:
If the battery is bad, your engine cannot turn over to start.
If the bearing that lets the crankshaft to turn freely are bad or worn out, the crankshaft cannot turn so the engine cannot turn over and run.
If the valves do not open and close at the correct time or at all, air cannot get in and exhaust cannot go out, so the engine cannot run.
If something blocks up your tailpipe, exhaust cannot exit the cylinder so the engine will not run.
If you run out of oil, the piston cannot go up and down freely in the cylinder, and the engine will seize.

http://www.automobileglobe.com/common-causes-of-dead-engine/