I would repeat whatever process you originally used to obtain 5 Star status. It may simply be that Hertz does not know if you still have the same Platinum card.
I tried that already and it did not work.
It generated another Gold Plus Reward but not Five Star. This was done at the Canadian Amex Platinum site after signing in and following enrol instructions.
It appears to be either a glitch in Hertz's system or this perk has been discontinued for Canadian Platinum members. The latter seems to be the case.