When you reference a .NET class that exposes events, and you switch on the WithEvents property, C/SIDE creates the event triggers for you. If you later want to update the reference to the .NET class, for whatever reason (like, there is a newer version of the assembly), updating the reference will actually delete the event triggers with all the code in them.
To be fair to this non-feature, at least it warns you politely:
This is not something you experience once in a geologic era. When you are developing your own assemblies, this will happen fairly often – as often as you add or remove events to/from your classes, and you want to reflect that in the Development Environment. Or as often as you increase the version of your assembly.
Unfortunately, there is no way in the Development Environment to update the reference while actually retaining the event triggers or code in them.
But still, there is a way, and a fairly easy way at that.