Pump unscrewed itself in the middle, how do I put it back together properly?

Hi all

When using my pump today it just sort of fell apart, and when I tried screwing it back together I had a few problems:

1. I don't know what to do with the little rubber ring
2. When I screw it up it seems like it pushes air through it but the end doesn't spring back enough to where it will hold itself in the pump holder thingy.

I can't seem to find any guides on what to do, I'm not sure if its broken or just disassembled. For clarity its from a C Line, but I don't know what year the pump is from because the shop lost the original pump and so sent me a replacement, I would its the pump from the past 2 years if that helps.

Thanks 🙂