If Google detects abnormal usage that may indicate that your address has been compromised, we may temporarily disable access. It will take between one minute and 24 hours for access to be reinstated, depending on the behaviour detected by our system.
Unusual activity includes, but is not limited to:
Receiving, deleting, or downloading large amounts of mail via POP or IMAP 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 signs in to Gmail.
Leaving multiple instances of Gmail 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.