The Google Collection can update itself silently without user interaction what could be an answer. (failed updates etc.)
But according to the releasenotes: Release Notes - Nik Collection Help no updates have been released this year.
Can you un- and then reinstall?
What do you mean by "can't use"?
Any error messages?
Menu items grayed out? Menu items missing?
PS tries to launch the plug-in but nothing happens?
I asked Google about the problem and they sent me a link to download the Nik Collection again. I was able to install over the earlier non-working files and now everything works.