Power issue

Hi 

My laptop goes off anytime i connect my power adapter. So if i am using battery and then i decide to plug in to the grid to charge, as soon as i do that the laptop screen goes black. after pressing the power on button it will then come on to the lock screen where i have to enter my password again. although all the information is safe it is annoying. i hope there is a setting fix that i can rectify this with.