laptop baterry charging problem

[bodgit]

I know next to nothing about computors and this is my first laptop a Dell inspiron 1545. new in june.
The thing has worked well untill last week I saw that the battery was not charging. I get the message...

The AC power adapter type cannot be determined. Your system will operate slower and the battery will not charge. Please connect a Dell 65W AC adapter or higher for best system operation.

The adapter that came with the laptop is plugged in, the blue light on it is on and the screen is brighter when it is plugged in and a check with a voltage meter shows 20.5v at the plug.
anyone know how to fix this please.