I just had this happen to me as well. All points gone. And the worse part: I did have a couple of Hertz rentals in the past 18 months, just not in the USA but only in countries that are 100% franchise branches which do not participate in their points program.
Let's just say that this is not the way to award customer loyalty. Even though I have been loyal to Hertz, have 5 star status and have been renting from them on several occasions in the past 18 months, they still took my points.
I'm pissed. I even took steps to prevent expiration - the text on the Hertz website (dutch version) on the reward account overview page specifically states: "Punten komen te vervallen als er geen punten verdiend of ingewisseld zijn in de afgelopen 18 maanden." - or translated: "Points will expire if no points have been earned or redeemed in the past 18 months". So I did make a points redemption towards a rental in Australia next week. But that did not change a thing, they still took all my points.
I have three Hertz rentals coming up for next month. I guess those will be the last - ever. I'm done being "rewarded" for my loyalty in this way.