Linux RedHat server work but monitor balckout after some time.

Sir I install Linux RadHad, It work properly & and manage all web based application, but when idle in some time and again login in server or by other PC through xmanager then at the time of login, user name and password display in screen but when give user name and password then screen blackout and no response even server work normally ie. run web based application through this server. plz help me ?
thanku