A Microsoft file hosting and synchronization service.
For those using OneDrive on a Mac, I have a solution. Basically, I just started from scratch.
Open your OneDrive via a web browser and log out (this is just to be safe but likely don't need to). For an extra step, un-share the drive you normally connect to and then re-share it. Again, I'm not sure this needs to be done, but this is what I started with in finding this solution.
You will need to unlink your Mac from the OneDrive. Open OneDrive and click the gear icon in the upper right hand corner. Select Preferences/Account the click on 'Unlink this Mac.' Then delete the OneDrive app as well as any associated files it has with it. If you have an App Remover tool, that should do the trick. If not, search for OneDrive in Finder to find these files.
Next, go into the App Store and download the OneDrive app for Mac. Once installed, open the app, log in and follow the prompts like you are setting up OneDrive for the first time. It should then log in to your account and begin the sync process.
I'm no IT expert, but my theory was there was an update or a fix made which didn't work with all the associated files of OneDrive. It essentially got confused and defaulted to the website. I have no way of knowing but at least this process got me back up and running. Been working for the past several days on my iMac and MacBook Pro. For you Windows users, I don't know all the issues that's going on, but maybe try the same process.
Best of luck.