at what temp does yeast die??? bottling question for beer?

i got 2 batches of beer that were fermenting and someone turned the heater off while i was gone for 2 weeks.
beer temp is now 40 degrees.
one fg should be 1.018 and is reading 1.020 at 40 degrees so i think it is pretty much done.
the other is reading 1.020 as well but the fg is supposed to be 1.013
i am gonna leave this batch since it probably isn't done and hope that it ferments down more when it reheats.

the other one is done and i wanna bottle it tomorrow. my questions is how do i know if the yeast is dead or not? if i bottle it and there is none in suspension it won't carb up.
any suggestions what i should do?
just give it a shot? or add more fermentables or something else????
help me please with any suggestions or advice.

Yeast don't die from the slightly low temps like you're describing,,,,they just become dormant/go into hibernation. Seems like you already realize that warming them up a bit will help get some yeast activity happening. Giving the fermenters a gentle swirl will also have a positive effect, just don't get the brew sloshing around (aerating) like crazy and you'll be fine. Then, leaving the brew to sit for a couple of hours before bottling should return some clarity to the beer again.

There could be a bunch of different factors, but I think your yeast will be fine (liquid yeast is stored around that temperature, after all). You may want to gently stir them up a bit to get the yeast back in suspension without getting too much air mixed in, let the temperature slowly rise back to optimum for your strain and let it sit another week. Test them after that and see if the FG has dropped significantly. If it has you may want to let it sit longer until the gravity levels off.

The other thing it could be is you may have had a lot of unfermentables which would account for the higher FG. Was this an extract, partial-mash or AG recipe? For an AG you may have mashed in higher than what you thought or your grain bill was low in well modified grains (i.e. low in amylase enzyme). If it was an extract, did you add a lot of unfermentables like malto-dextrin or lactose and didn't account for the gravity increase?

Either way it goes I figure you'll be fine if you give your batches some extra time to catch up.