But suddenly when I started laptop in the morning it showing black screen. (No display), (Not even showing dell logo or bias set up screen, totally black screen as in power off state), Whereas power button is on &fan also working, but no display.
Do a hard reset first it may have gone into Hibernate Mode. Remove the battery(if not built in) and AC adapter then press and hold the power button for 30 seconds then try and boot it back up. If doesn't boot up try connecting and external monitor first and see if you'll get any display from it before you go ahead and open the laptop to reseat the RAM.
Reseat the RAM if the first 2 steps didn't work making sure that they are all seated properly into each RAM slots. If all these steps failed it may be a motherboard issue.