Floorboards have come loose
Just align everything and tap it back into place with a block of wood laid over the top. Work slowly and watch for misalignment, working from the end that's still in place. 

It should go back in without much trouble unless it buckled because of severe swelling due to moisture. In that case, wait until it dries out and try then. 

I'd use galvanized, coated, or stainless steel trim-head screws to re-fasten. Nails may not hold well in that old framing. Screws with pilot-points will help prevent splitting.