A number of things can cause an engine to lose compression, and just about all of them involve mechanical failure of some kind. Either the engine is out of time as described above or there are bent or broken internal parts that bleed off compression. So, bent valves, a hole in the top of the piston, or broken piston rings can cause a loss of compression that can equate to a no-start condition. To check for this, run a compression test. Here's a video on how to perform that.
If you have low or no compression, it's good to know the cause. For that, you could do a leak down test to find out where your compression is going. Here's a video on that procedure.