[qazxsw]

I have Dell Latitude c840 and when I press power button all three lights is turn ON for 3 to 4 seconds after that get OFF back and laptop does not work.
Can any suggest a solution?

[floppybootstomp]

99 times out of a 100 the symptoms you describe are caused by a faulty battery, even if the Mains/DC adapter is plugged in.

Try buying a new battery.

[philieans]

I guess Dell will call back the d840 battery, Dell ever recall batteries for blinking issues for 2 times, I remember. If the laptop is still in one year warranty, Just ask Dell CS for advice.