I have been baking sourdough bread for a long time. The first few loaves were leavened with yeast, but then I switched to a sourdough starter. I soon got another starter, but that one soon fell by the wayside: I got a couple of loaves out of it, but it soon began to smell like baby vomit, a not uncommon problem. I tossed it. But the OG starter survived, and I used it for the next 15 years. Sometime last spring, someone gave me another starter, which I revved up and started using, as well. It was more vigorous than my other one, which had been slowly declining. My practice was to bake one loaf per week, putting the starter in the refrigerator in between uses. I'd feed it, then a few hours later take off what I needed, and put the remainder away. The decline was pretty rapid once I got the other starter. I was alternating weeks, so each starter was fed every 14 days or so, then put back in the refrigerator. Soon, both starters were pretty weak.
Doing two feedings on the new starter worked pretty well, and I started using it most of the time, eventually idling the OG starter altogether. I did some reading and I realized I was doing a lot of things wrong. There are a number of websites around on managing and reviving starters. I should've been feeding the starters just before refrigerating. I will do that going forward.
After the OG had been in the refrigerator for a few months, I hauled it out and fed it. And fed it again.
The above picture was taken a few hours after the second or third feeding. It was not quite flatlined, but there wasn't much life. I continued with the CPR, switching to 2x daily feeding (and discarding, to keep the jar from overflowing). The house was in the 60s, and I also put it in the oven on proof for a few hours (temps when the oven is running on the proof cycle go as high as about 116, but are usually about 95-105). Finally, after several days, I got this:
It is doubling well and has the normal lactose-tangy aroma. No baby vomit. It's actually showing more life than my other, newer, starter at this point. Going forward, I think it's apparent that using them once every couple of weeks isn't enough--or at least that if they are not used for that long, feeding just before refrigeration (and maybe weekly while refrigerated) will help.