I'm sorry for the loss of your snake. It could have been many things, and all we, or the vet over the phone can do, is guess. If not too late, you can take your snake in for a necropsy which may reveal what the cause was, which may be helpful if it turns out to be something that can affect your other snakes.

For a herp vet in Europe, begin looking here https://www.eczm.eu/Herpetology%20Specialty.asp
You may have to call around and get recommendations that are closer; but being a reptile you may have to expect to drive a while; especially if in a very rural area.