Site down, then demolished - AWS


I am using amazon linux 2 instance (t2.micro).

I have installed a simple php script on my server.

Last time, I was clicking a button in the backend (‘create new content’; which I used dozens of times before),
the site froze.

Nothing worked or moved anymore.

Then site was down (not reachable).

After 20 mins, the site was back online .- but it had completely changed:

The blue stlyle colour which I had in menu, footer aso turned into dark grey / black.
All content - pics, vids - are gone
All user profiles are gone
I cannot log into backend, as it says password credentials are wrong.

What has happened here?

Does anyone know or can guess, what may have caused this?

A developer I know says its a server side issue, as the php script works correct.

Thanks for any help,