I had the same, nothing worked, until I changed the start-up setting of my DSN from manual to automatical.
this can be changed after pushing the small DSN icon at the middle bottom. no problem since then.
bert
November 2010
i take it you are using a DS? if so take the cart out and blow on the connectors (crazy it sounds but it works) and dont use an eraser use a little alcohol on a cotton wool bud and wipe this over the cart connetors
jason
January 2010
It was probably just dirty, but by "cleaning" it with an eraser you might have ruined it.
Anon
January 2010
I can answer this question...
My nintendo says no Game pak inserted when there is?