There could be over 1000 reason. You can google OBDII Codes and you will see.
It could be as simple as a loose gas cap but there could be issues with sensors, control modules, wiring problems, mechanical problems, etc...
Last year when I first posted the question, no code shown. Now the code showing is P0100. Suddenly having problem with getting engine started. difficult to start the engine, but once the engine is started runs well. Why is it happening?