I have whipping cream that has been frozen; it is now separated and won't whip. Is there anything I can do to make it whip?
Whipping cream that has been frozen is perfectly fine to use in many dishes, but it won't whip.
As far as I know, this is irreversible, so no, you can't make the cream "whipable". If you are very lucky, you will get it to a sort of semi-stiff stage, but nothing that would be suitable for cake decorating or similar.
Not part of your question, but possibly interesting for next time:
Whipped cream can be frozen, so if you have leftovers, you could pipe little tufts or similar, freze them and use them to decorate desserts.
There seem to be mixed experiences of whether or not frozen cream can be whipped: this discussion thread has voices on both sides. Note that those who say it can be whipped talk about possibly needing to re-emulsify it. If I were to try to whip frozen cream I would add some soy lecithin to try to help with the emulsification.