Simply visit https://www.blizzard.com/en-us/

Click "Download Battle.net" in the top right corner - done!

I've seen people considering reinstalling their entire clients but there is zero need for it. Your Launcher failed to auto update (firewall related I'd guess) and manually downloading the Launcher will fix the problem.

holy f*ck thank you!

I was very relieved as well lol. Cheers

Mines eventually fixed itself somehow.
But thats great if it works with a simple click.

That's good! Maybe they got feedback and made adjustments. We weren't the first ones experiencing this issue. Some have had it in earlier patches too.