phone erases itself!!

I've never heard of that, but perhaps it was hacked

Maybe it was a software glitch?

Is your S9 rooted? Because that's the only time I've seen a phone resetting and clearing itself, with a rooted phone running a custom ROM that was buggy.

"Hacked" is probably the least likely explanation (it's a conclusion people often jump to when they see something they don't understand, but usually there's a more prosaic explanation).

I don't know about the S9 specifically, but we have over the years had occasional reports of spontaneous factory reset from a number of different members. Sometimes it follows some event like the phone running out of power and turning off, but not always. So it's very unusual but it's not actually unheard of.

Unfortunately when the only symptom is that the phone is wiped it's very hard to identify the cause. It could be a glitch during a system update, especially if you have one of those phones where the carrier can push the update without requiring your consent, since then it could happen without your knowledge. Or it could be the result of some hardware glitch or fault. But we've really nothing to go on to identify the cause.

If you want to persue the "hacker" line, check your Google account (and Samsung account if you have one) for any unrecognised activity, such as logins from devices you don't know (logins from places you haven't been to are harder to judge, since they get that from my internet provider or network operator's IP address rather than the actual location of my phone or laptop, so logins on my Google account often appear to be from my device in a place I've never been). If you see unrecognised logins you should secure your accounts anyway, and deny access to those devices. The reason I'm saying to check this is that your accounts are the most likely way into your phone. But I think it very unlikely that this is the cause.

Hadron said:

"Hacked" is probably the least likely explanation (it's a conclusion people often jump to when they see something they don't understand, but usually there's a more prosaic explanation).
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Click to expand...

A statement that needs to be emphasized. Once you just assume the problem is due to '...being hacked' than you'll never find out just what was the actual cause. Odds are when your data was spontaneously wiped in the middle of the night, it was because of some seriously bad software glitch.
Theoretically, if some mythical 'hacker' did manage to compromise your phone, he/she would be doing so to do some kind of data mining on you. Deleting your data would be just an act of pure malice, the issue being there's the TV/movie version of a hacker, and then there's reality. Also remotely accessing your phone in this situation would be daunting task, something that requires an advanced skill set, and they're targeting you specifically for whatever reasons. (... unless you live with someone with easy physical contact to your phone, and who is really pissed at you, which would be more of a family politics matter.)

Anyway, this isn't a common problem and determining just what happened isn't going to solved on an online help forum. All any of us can do is speculate and guess. This is more of a hands-on issue to determine just what happened, someone at a trained, technical skill-level to do a basic forensics examination.

On a side-note, do you have backups of your data? Hopefully yes so you lost a minimum of important stuff.