a-dinos It sounds like "my team mate said MYSQL is not starting, he tried to solve it, after many config changes," has resulted in a change to MySQL that osTicket isn't expecting - table/database name, username, password, etc... The vast majority of the settings are stored in MySQL, so make sure the details that are in ost-config.php are still correct, and make sure you can connect to MySQL from the command line using said details.
If anything has changed, update the settings and you should be back to normal - then nag your colleague about undocumented changes 😇
EDIT: Oh, and once you're back online, schedule recurring backups. Hindsight is a great teacher in cases like this 😃