Is absolutely any server crackable?

Id like to know what are the main advanced methods of cracking a server, as a list, for the purpose of protecting our servers better. (ie. bruteforce, sql injection.. may not be that advanced ) For example, if i had a simple server responding with a static file on port 80 and strong passwords, would that server be crackable ? Some say firewall is useless, but assume everything had a basic configuration.

Im asking this question because I just found a cracked website that I use from time to time, its front page mentions the name of the team who cracked it. I searched the name of crackers and I found many other websites or pages cracked.

This website I accessed was one of those website with tools where you submit a domain and it gives you details about the website. The owner must have much more experience than myself, and Im not even sure if he knows. Maybe I should check again and try to contact him. I dont know how to offer rescue in such a case..