OSX 10.6.8 not showing login screen after boot

Hi, I've a problem with an Xserve with SnowLeopard server mounted inside (v. 10.6.8)

When I boot my xserve appear the apple initial screen but after that will not appear the login screen, but instead i have a light blue screen and no more.

I've the access with ssh and no more, because i can't also load the "sharing screen" utility from command line (I've userd that guide https://discussions.apple.com/thread/2332183 ) and I don't know why.

I've also tried to reset the PRAM without any resault, anyone can help me?

Thaks a lot!

Gino