I just removed a setup that has been there for over 10 years. The carpet is crushed down and I wanted to know if anyone had any tips to fix the depression. It seems it happens with the furniture as well.
The underpad is probably destroyed, and won't come back, but you can try to pull the pile back up, as mentioned above. Even wetting it and brushing it up will help, but it is unlikely to regain it's full form, IME.