How to deal with a complete failure in a group research?
Failure is intrinsic to science: how can you think of exploring the unknown without ever failing? Alas, it's a certain modern, distorted, entrepeneurship view of science that brought us thinking that science should always be successful.

For my master's thesis, my PhD thesis and part of my subsequent research, I worked on an experiment that eventually didn't work. Ten years. But I learned a lot, and now that more than ten years have passed from the moment I realized the failure, I don't regret it.

So, don't be discouraged and focus on what you have learnt and not on the failure.