If there is still some warmth, like it's not room temp already, I would put some sort of lid or cover on the tank to preserve as much heat as possible.
Desperate time, desperate measures. Mayve pull out some water and microwave or heat on a stove and re-add? Someone chime in if this is a terrible idea for reasons unbeknownst to me.
fwiw I keep my tank at 78 (check it with a few diff thermometers) and when I stick my hand in the tank it feels cold so try to get the actual temp of it
Thank you to every one especially Cell. I microwaved some water and covered it to preserve heat. Thanks to everyone for all your great ideas! They really helped a lot.
get a spare heater...they have always been the most replaced piece of equipment on my tanks...many folks recommend replacing them every winter whether they are working or not...i've bought many brands (usually stuck with what the lfs or petsmart has in stock at the time of the failure) and can honestly say they are all garbage