Losing someone sucks. Especially when they were an important part of what you did. And you can't do anything about it. They're gone.
But the saying, "One books closes and another opens" rings true here. Losing them means you have lost some of yourself. Now, you have the chance to build up yourself again. And what you decide to build up again might be better than what was before.
So, if you lost someone, get ready for the work ahead of you.