Despite living in a similar climate (we have about 6-7" of snow that just fell) I'm not sure what you can do. It could be the liner, but it could also be a leaking plug if you have a bottom drain, or a cracked pipe underground. When your pool freezes (as it may already have) at least the ice can keep the liner from floating out. How much of an ice layer do you have?

I would still add water, which is a pain as you must open a hose bib, then drain it again (or get a foam cover for it).