Fatigued yeast?

[left field brewer]

I brewed an ESB last week and fermentation was going smoothly(less than 8 hours lag), putting the fermenting beer in a water bath inside a cooler. On the third or fourth day, however, I was on my way to work and checked the temp and I had let it rise up to like 67 accidently. I added some frozen 2litres of water to the bath to bring it down, but because fermentation had slowed it wasn't producing as much thermal energy. So I think the brew got too cold after I went to work, and the yeast fell out of suspension. How cold would it need to get for that to happen? Best remedies? I took a gravity reading and it is only at 1.020, and seems like it has almost stopped. The O.G. was 1.054, and the yeast is the Rogue Pacman(4th gen). The yeast has never really been washed, I usually make a brew and then pitch on the yeast cake. After that, I save some of the cake in flip-tops and refrigerate. The last time I used it before this(IPA), the primary sat for about a month before racking and saving. Does the yeast wear out and attenuate less over time? Thanks.

 

[Catfish]

I would guess that the yeast got too cold to quickly. You can try letting the temperatures rise again and some of your yeast may get back to work. Try swirling them back into suspension once the beer has warmed up a bit.

 

[seawort]

hmm... edited as I actually read all the way through the post this time...

looks like you may very well be simply done... thats an awfully high FG
does it taste sweet? is it possible that you were mistaken about your hydrometer reading?
otherwise... yeah warm it up and swirl that pacman back into suspension. It seems quite unlikely that it is fatigued.