I have an 'Energizer' brand replacement battery for my Nikon camera.

Model ENEL9
Li-ion 7.4v 1000mAh (7.4Wh)

Left it in the camera unused too long and it is flat and won't charge.

Anything obvious I can do to revive it, or just dispose of and replace?

I still have the original battery so the camera is working.

Cheers

Dave R