In the RSU agreements I've seen, what you're "really" being offered is some number of shares. The dollar amount that gets quoted is based off of the stock price at that time, but that's not usually guaranteed. So if it was trading at $100 / share when you were hired, they were agreeing to give you 1000 shares. If the company stock goes down to $75, you should still be owed those 1000 shares, vesting 250 of them after one year then more as time goes on.

The fact the company went private can certainly change things. One the one hand, your RSU agreement may stand, but you just receive shares in a non-public company. That usually makes valuing them more ambiguous, and can limit who is available to buy them from you. Or there might've been some "buyout" of outstanding shares (vested or unvested), but you should've gotten some communication about that. Or maybe going private involved invalidating any RSU agreements, which would definitely be a dick move, but might be allowed.