I was told some years ago, if you put cotton thread in a ziplock baggie with a spritz of water and place it in the freezer for a week or two, it will be good as new, The cotton thread dries out over time, and rehydrating it brings back the strength.
I have done this for years, and am using cotton thread that is over 30 years old (from my grandmother) without any problems.
Try it, you don't have any thing lost if it doesn't work, and a lot to gain if it does.
Mldesatnik