Dell precision m6400 won't start with 210w charger

I bought a used 210w ac adapter on ebay for my m6400, and it's been working great all day. Just a few minutes ago I turned around and my laptop was off. When I went to turn it back on again the light on the ac adapter and lights on the laptop blinked and then wall went out. If I unplug the adapter and plug it back in it will light back up, but then immediately turn back off again when I try to power the lappy up.
Any ideas as to what could be wrong? This is my only laptop and I'd prefer not to lose it.
Any help is appreciated ^^