Cranks but does not start

In morning my car does not start and the battery get low
wen t start then no problem on the whole day
and I have a doubt that thus I can change the latest engine

The ignition comes on and when you try and start the dash lights and radio go off but no sound from the starter like its trying to start - nothing just dead.
Removed the ignition switch and tried and some times it starts and other times it refuses,

This problem is driving me nuts, some times it starts every time and then other times nothing.

Please help.

Last week I changed the heater plug and also checked the battery voltage 12.6v

It sounds like your car battery has a dead cell. Make sure that it's fully charged and have it load tested. Also make sure that the terminals and cables are clean and tight on top of the battery and the battery does not have any green fuzzy stuff growing on top of it

Thanks for ur reply
but I changed heater plug last week only

It's a diesel 1.8d ford ikon. Heating plugs and battery has been replaced new by service person. Its taking to long when I start it the first time in a day, engine is cranking. Once the engine is started in a day, it's working fine and starting within 5 seconds. But if the car is stopped for more than 6 hours, it's again taking time to start. Mechanic says its is engine week problem do to engine, is it true please help any answer as to what I can do.