The title might not be as descriptive as I would like it to be but couldn't come up with a better one.

My server's file system went into Read-only. And I don't understand why it does so and how to solve it.

I can SSH into the server and when trying to start apache2 for example I get the following :

username@srv1:~$ sudo service apache2 start
[sudo] password for username:
sudo: unable to open /var/lib/sudo/username/1: Read-only file system
 * Starting web server apache2                                                                                                                                                                                                               (30)Read-only file system: apache2: could not open error log file /var/log/apache2/error.log.
Unable to open logs
Action 'start' failed.
The Apache error log may have more information.

When I try restarting the server I get :

username@srv1:~$ sudo shutdown -r now
[sudo] password for username:
sudo: unable to open /var/lib/sudo/username/1: Read-only file system

Once I restart it manually it just start up without any warning or message saying something is wrong.

I hope somebody could point me into the right direction to resolve this issue.

Thanks in advance!