It happened to my website.
It's a bug or something,
it happened to me after a VPS reboot.
The database became corrupt.
Fortunately, I had a backup!
Your domain name must be pointed at your VPS IP: 62.171.131.74. Head to your DNS manager and add an A record, then try again.
It's probably due to a problem with caching.
No, Roundcube is not strictly required for email, but it provides a user-friendly webmail interface. You can manage email via the terminal with tools like EXIM4 for sending/receiving messages if you prefer a command-line approach.
You can find how to fix it here, I had the same issue and resolved it this way:
https://www.youtube.com/watch?v=AHYIc0Px-X0&lc=
For the complete tutorial on how to install a mail server on your VPS, you can find it here:
https://www.youtube.com/watch?v=ituA0HpaxuM&t=32s
https://www.youtube.com/watch?v=ituA0HpaxuM&t=32s