I had this recently happen to me about 2 months ago. I used it to glue something together and the next day it was rubbery. I look at the bottles and one has a cloudy bottom. They were new bottles purchase earlier in the summer.
I have some 5 minute jb clear epoxy. I bought it to repair really shitty $35 house cues for the local pool hall. They are animals and crack and break the ends off all the time. Once I fix them they are stronger as they were hollowed out to maybe be LD or something... with a fat conical tapered house cue with absolutely brittle wood.
Anyway, the resin was about 6 months old and starting to be cloudy. I googled and read somewhere you can heat it up and mix it. I brought water to a boil in a pint jar , took it off the stove, and set it in there for about 20 minutes. Stirred it with a bamboo skewer. They say to let it reach room temp before using. It cleared up well and got to the normal consistency. It worked really well. I wouldn't necessarily trust it for work on a customer's cue but it did work and no problems with the cues.
There is mixed info, some say they've done it to 10 year old epoxy and it was fine, others say it might not be the same strength. It worked but again, I don't think I'd do it on paying work, these were just done for charity to help the hall out and get practice at repair work.