What can happen to your home brew if you do not rehydrate your yeast before pitching?!

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You shouldn't need to rehydrate yeast, just add it direct from the packet.

As for advice, that's going to be a bit difficult as we don't have information we need.

It could be that it's too cold.

It could be that you didn't have enough sugars in it and the yeast has used them all up.

You could have an infection that has killed the yeast.

All of this is guesswork as I have no idea what equipment you are using, what you are brewing, and how you are brewing it.

Luckily my 5 gallons of IPA are clearing nicely, and the 5 gallons of cider that will go in the pressure barrel after that is drunk is bubbling away nicely at fairly constant temperatures around 20C

If you are using dry yeast, just pitch it into the wort. It will rehydrate there. Some people do make starters but I don't thing the added speed in fermentation is worth it, provided you have proper sanitation.

My best guess (and that is all that it is) is that your initial stage of fermentation is over. Leave it in the primary bucket for 2 weeks and then rack it to a glass carboy for another two. Then prime and bottle. Two weeks later, cool one, open it, and drink it for me.

It sounds like you should invest in a hydrometer.