If the yeast does not work, it is usually a sign that you have too few active cells.

Yeast is a sensitive organism and under certain conditions it stops working quickly.

This can have several reasons:

1. the pitching cell count is already not right, i.e. you have too few dry yeast packets to the amount of wort prepared (usually does not apply to construction kits).

2. the yeast was damaged by temperature, e.g. the yeast package was left on the sunny windowsill or next to the hot kettle.

3. the yeast package was damaged and draws air. Oxidation can reduce the yeast cell count over a longer period of time (rather rare).

4. the pitching yeast was exposed to stress (temperature fluctuations, osmotic pressure/too high original wort vs. too few active cells). In this case it is often the starter that you may have started, which was warmer than your pitching wort, then just 2°C lower temperature is enough and the yeast gets a cold shock. Therefore always the same temperature of starter and pitching wort or transfer the yeast into a warmer environment.

5. the yeast was not properly nourished to be able to multiply or therefore cannot produce enough enzymes to metabolize the carbohydrates (e.g. zinc deficiency). The case is unlikely with a pure malt wort, which feeds the yeast well. Now, you may have made a starter with water and waited too long to pitch, as the yeast will start to settle after just 30 minutes without food.

6. insufficient aeration of the wort. If the amount of yeast packets is sufficient with the amount of wort according to the recipe/yeast specification, the wort may have received too little oxygen before pitching with the yeast. The yeast multiplies significantly more effectively under aerobic conditions and the brewer needs a lot of young cells for fermentation, so aeration (actively by sterile air supply or passively by "letting it ripple") is important.

The last point is therefore also your chance to get the fermentation going again. If you let the pitching wort splash once again from one vessel into another or stir intensively with a sterile cooking spoon, you will get the propagation going again and after some time the fermentation will start again.