I whipped some cream with soft peaks and put it in the fridge. A few days most of it has collapsed and is liquid. Can I re-whip it effectively without adding more cream?
Sneftel answered the question you asked, but to answer a corollary question: you can prevent this from happening by using a stabilizer (also called a cream stiffener). One you can get is by Dr. Oetker and is called Whip It (or Sahnesteif). There are a few other alternatives as well.