Backlight goes dim when ac power is plugged in

Hello,
I have a Lenovo Thinkpad T420 laptop. at first when i plugs its power supply the display would go dim, means there will be no backlight while there is display on the background. that was and always had been resolved when i closes its display and opens it again. but now it refuses to do so. now when i connect the power cord the diplay would go off(as mentioned above). when i unplug it also, the condition prevails. then i have to close the lid and open it again to access my desktop with bright screen. also i noticed that the refresh rate changes when i plugs in and off the power cord. it changes from 60hz to 50hz. please help!Any help will be appreciated.

Thanks