HP Black screen when power on...

hello, i have hp pavillion 2120dv...when i press power button without plugin chargin adaptor...it is starts bt screen is black no process..when i put power supply by chargin...it starts bt same problem but with external power when i press button and also i have to press enter jst for 6 or 7 sec than my computer starts....than if i will unplug charger...it is working properly. Can u plz help me out of this problem..i am really worried..! Thanx