Hello,
I'm totally hopeless. I don't know what to do. My pilea stopped growing about two months ago and since that, plant started dropping leaves. It is on a good place, not over-fertilized, nor over-watered. Please help me. I don't know what's going on...

Yours is a very tiny plant with a tiny root system. It is in a relatively large pot with what appears to be a heavy potting mix that lacks porosity (perlite). All of that adds up to a strong likelihood that the roots are not getting enough oxygen. At this point, repotting and replacing the soil would probably be too traumatic, so I recommend that you allow the soil to dry deeper into the pot before adding a small amount of water. Keep it warm and in a sunny location. Do not fertilize it.

Thank you very much! I'll put it on a window. By the way, the potting mix is only peat. And yes. I didn't add any perlite to it. How often should I water the plant?

Allow the top quarter of the soil to dry out before adding just enough water so that it reaches that level of dryness again in about a week.