The starter (containing wild yeast and good bacteria) has finished fermenting so it gets hungry and exhausted.
It deflates because the fermentation stopped and no air is being produced. After you add more flour, you are feeding the bacteria and they produce air and it will rise again.
The temperature will affect the process. It will be slower to occur in colder climates and much faster in warmer places.
If this happens,
- Just leave it for 24 maximum for the first 1st rise.
- Then for the 2nd, add a little more flour if needed and then 'fold' the dough quite a few times to trap air inside.
- Then leave for another 2 hours.
- After this, it should grow and also the once in the oven, it will rise again.