Do you have an old, no-longer-in-use Google account? Maybe you switched away from Gmail years ago, or maybe you just have a previous account you haven't looked at in a while.
In either case, you'd better double-check and back them up if they have any information you want to keep. Users on Gmail's support forums are reporting receiving messages in their backup email accounts saying their old Google account is kaput because of a terms of service violation.
These users are complaining that they should have had some sort of warning and opportunity to revive old accounts before Google arbitrarily deleted them. Moderators on the Gmail forums haven't heard any official explanation from Google.
Nonetheless, if you have one or more Google accounts that have been inactive for a while and you'd like to retain their contents, you should do so now. Go to Google Takeout and click "create archive" to download your data to your hard drive.