In my case, my lockout had nothing to do with any of the activities described below, but rather a stuck key on my keyboard. The key had stuck down and I didn't realize it until my Mac started to beep repeatedly. I fixed the key but was presented with the message below.
It is a little distressing that a rather simple computer malfunction can result in a day-long lockout and the fact that there is no appeal process for such a lockout.
This is the first issue I have had since switching to Google, but it now makes me a little worried about using it.
Here is the message from Google...
Account Lockdown: Unusual Activity Detected
This account has been locked down due to unusual account activity. It may take up to 24 hours for you to regain access.
Unusual account activity includes, but is not limited to:
Receiving, deleting, or downloading large amounts of mail via POP in a short period of time.
Sending a large number of undeliverable messages (messages that bounce back).
Using file-sharing or file-storage software, browser extensions, or third party software that automatically logs in to your account.
Leaving multiple instances of your Gmail account open.
Browser-related issues. Please note that if you find your browser continually reloading while attempting to access your Inbox, it’s probably a browser issue, and it may be necessary to clear your browser’s cache and cookies.