An update for Chrome OS, version 91.0.4472.165, has left some users out of their accounts (login bug) or according to information on boot-looping computers. If you're logged out (login bug) from your Chromebook, here's the solution: Google posted on system status page that he is working on a correction, which is expected to be released on Wednesday. He also withdrew the update, preventing the problem from appearing to more users.
According to Google, the only options for instantly regaining access to your Chrome OS device (if you unfortunately downloaded and installed the update) will be to delete the device's local data. Your device should automatically download this update to resolve this issue. Alternatively, you can activate the update manually using the guest account. If your computer has downloaded the "bad update" but has not installed it, you may need to avoid shutting down or restarting until the repair is released.
If you can't wait for the firmware update, Google recommends either "Powerwashing" your Chrome OS device, resetting the update with a USB recovery key, or deleting your account and reinstalling it. Again, all of these solutions will erase local device data, so be careful. The Google status page does not address boot looping issues - Android Central says that if you experience these issues, you will need to use the USB recovery key method to reinstall Chrome OS.
Broken Update 91.0.4472.165 is actually the second Chrome OS update released this month: Google withdrew another update around July 7 after users discovered that some Chromebooks were having serious performance issues.
Source of information: theverge.com