You know first that wood is like a sponge. In the winter time, the home's unit will dry out the air and cause the flooring to actually shrink to an extent, just the opposite happens during the Summer.
Your problem sounds more like loose boards in a run that just came loose. Expansion is all around the walls under the quarter round molding.
If and only if the loose boards are close to the wall area, you can remove the floor molding, and then begin to pry the boards over to fill in the gap. This is a slow process, since you have to move one board at a time and work to the loose board. Some will be nailed down and others will be free, but locked in to each other.
Now, if the loose boards are in the center, then its almost impossible to fix this.
There is some colored to your flooring putty that you can force into the cracks and allow to dry and then sand it in and then stain it and treat it just like your existing flooring.
They key is to make sure the putty you get is STAINABLE. Most putty is not, so be careful.
I hope that you are able to force the boards tight up agains the loose ones as that is the proper manner of fixing this.