In general, with the report made in Blogger Help Forum: Get Help with an Issue, we may dismiss the owner as a "hacking victim". We may suspect just one more botnet in operation, stealing peoples accounts and blogs - and the victim unwisely revealed his email address.
Sometimes, the story is more bizarre.
When a blog owner forgets or loses login details, we'll try to research owner profile information, by examining blog source code.
With blogs using dynamic templates, and with deleted / private blogs, owner profile information can't be easily found. There are still more reasons why online diagnosis cannot help everybody.
If we can't help a frustrated blog owner recover access, he may try other solutions.
When we can't provide useful hints - or research blog owner profiles - a frustrated blog owner may resort to alternate techniques to recover account access.
Generally this will involve making a list of all email addresses used around the time the blog was started - then trying to login to Blogger, using all possible passwords for each email account, one by one.
Brute force blind recovery, with luck, may be successful.
With enough determination or luck, this may produce success - and control will be recovered. Sometimes, recovery comes - at a price.
We've seen a few reports, recently, about demographic details being requested - with the requirement to list recent Google account activity, or when was the account first used?
When unsuccessful, repeated attempts at logging in to a given Blogger account, using the wrong passwords, can cause "suspicious" / "unusual" activity account lock - and other blogs will go offline. With the would be blog owner logging in blindly, the effects of the repeated attempts may not be observed, directly or immediately.
An account locked for "suspicious" / "unusual" activity can produce offline blogs.
"Suspicious" / "unusual" account activity lock can affect innocent third parties.
The account / blogs affected may be owned by the person trying to recover control - or possibly by an innocent third party.
Long ago, people would receive mysterious email. Now, we have mysterious locked accounts and deleted blogs - involving an account name being "guessed" by another blog owner, and multiple bad password attempts.
Some hacking victims do not cause their own troubles.
All hacking victims are not innocents, who naively disclose their email address.
If your account was locked, for suspicious / unusual activity, you'll have to request Appeal / Review, and follow instructions. And you may have to wait out the locked blog review process too.
This scenario is yet one more reason why you need to use Google 2-Step Verification.