Laptop is not starting

hello,
before 4 days ago ,i shutdown my laptop properly.and next day when i was start my laptop it was not starting.i remove battry and power cord and press power button for 20-30 second,then reinstall battry and power cord and start laptop.but it is not starting.next day again i start my laptop ,then it start in normal way.but next day again it was not starting.some time it start and some time not.
what is the problem.i use dell insprion i3.