iMac Unable to connect to network account to Mac mini server

Running iMac Moutain, on MacMini Moutain Server for 1 month....

Yesterday, when a came back from lunch.... Unbable to unlock my screen.... So I have to turn off the iMac. To day same thing... Sometime it work, some other, I have to restart my iMac.

But this afternoon, Unable to log back on my network account. Always a red light...

I change the network cable... still not work

I reboot, still not working

I try on another iMac, I able to log in my network account...

Any of you guys have a idea ????