Same thing with me today. Of course if you try a password reset with a locked account it says "Try again later", so that doesn't work.
Called in and did the form. Agent stayed on the line and reviewed the form. I was back into the account in 15 minutes total. When the account is unlocked you reset with a new password. Agent said she had a record number of reset requests yesterday.

This is probably an organized hack (due to recent international events) where they just flood the system with logins for email addresses on their list. Not sure anyone is really trying to gain access to accounts, just overwhelm the system. Hawaiian airlines was hacked yesterday and was down for many hours. At least this "effort" has not taken down delta.com up to now.

If you have an older email address that has been on hacked lists before (probably most of us), then this might happen to you. If I was flying in the next few days, I would check my app every day to make sure I can still login.