Were you using a CDP/corporate rate? Does your email confirmation say "Rate is guaranteed"?

Corporate rates generally are NOT guaranteed. If the company re-negotiates a new rate with Hertz between when you book and when you pickup, or if the contract is cancelled/expired, then the rates will change to the new rates. Hertz is not only within their right to change the rates in a case like this, but it's exactly how it's supposed to work (including if the prices drop).

If it's a public rate, the reservation email should show "Rate is guaranteed" on it - in which case obviously this shouldn't happen...