If you're determined to restore it yourself, one shortcut is to add a new coat of varnish, in the hope that it will "hold together" what's already there. Another option is see whether the varnish is removable and replace it. Try some solvent on a tiny bit of the painting in an corner (even better if there's some paint on the edge under a frame where it isn't usually seen). Just remember, it's easy to make a mess, you can disturb the actual paint, and you can end up worse off than only cracks.
See Also: How Do You Restore a Pollock Painting?