Why might the site crash?

Question

Yii

In general, there was an incomprehensible problem.
I worked on a portal written in yii2 for about six months. The day before yesterday, it suddenly crashed, nginx began to issue 504. All subdomains also went down. I rebooted the server, everything worked for a minute, then it crashed again.
There is nothing unusual in the logs, traffic has not increased dramatically, no changes have been made to the functionality.
Moved to a new server today, same story.
What could possibly happen?

Answer 1

Figured it out thanks to the support. There was a ddos ​​attack. Turned on protection, everything worked. Thanks to all.

Answer 2

Is a version control system in use? Perhaps the hackers changed some files by introducing their piece of code there, but it does not appear immediately, because. there are calls to some third-party site.

Answer 3

anything could happen
hire an admin or a programmer who would look at the logs and solve the problem