Hi:-)
There's no lamp inside a plasma TV to check, the plasma pixels generate their own lite on the display panel.
First try plugging the TV in another wall outlet, be sure your using an AC surge suppressor to plug your TV into. If your not sure what an AC surge suppressor is then
click here. Surge suppressors protect the TV from harmful AC spikes that can damage the TV's power supply circuit over time. These AC spikes come from the wall outlet!
If all the above is done and tried with no results, then read below:
This symptom indicates a failure on the power board inside the TV. Or, it could also be logic circuits holding down operating voltage due to a fault outside from the power board. There's no quick fix on this problem here, the TV needs servicing. If its under warranty take it back or have your local repair shop fix it for you.
I hope this sheds some light on a solution for this unfortunate TV problem.