UNIX for Advanced & Expert Users

Hello,
I am afraid that it might be difficult to solve your problem here.
Your administrator did something - let him work on it... it might be a problem on his side - if not then he should be able to tell you what is happening.
If this is about the connections then your administrator should be able to read the logs (ex. who, when, from where, how, with what user account and password tried to connect) - you should not be allowed to read that as it would be a security issue.
Good luck

Even the admin in my case does not know how to find out. Which logs would tell him this information?

How are you connecting to the server? I hope that it is SSH or something like this. Check /var/log/auth.log. If your admin did something (I mean the thing that blocks your connections) then your admin should have some logs and know the cause why your login was blocked. If not - I would think about another admin....
Good luck