Hi Emmanouil
I'm surprised you did not find anything when searching the forum? It's been mentioned many times before. Perhaps this will get you going again?
http://support.apple.com/kb/HT2129
The drive will be wiped if you decide this is the option for you. Unlike OSX Client there's no archive/install with OSX Server.
Bear this in mind when configuring your server again. A slack/so-so/unconsidered decision made at the Setup Assistant phase can/will have major consequences sooner or later. Some of which will be difficult to 'cure' without a reformat/reinstall. One word of advice and three rules: Begin at the end to start at the beginning. Yes I know but think about it and it does make sense. The rules: (1) DNS; (2) DNS; (3) DNS.
If there is something of value on the Server target disk mode it first to another mac. How do you Target Disk Mode? Attach a keyboard, reboot and hold down the 'T' key. Wait until you see the drive(s) mounted on the other mac. Copy relevant data. When done eject the disk(s), disconnect the firewire cable and follow the support article.
If you have another server you could create an nbi of the Server Installer Disk and start again that way.
HTH?
Tony