Most common issue is the user has typed the wrong password and the device has been varied off. You can check this by doing a DSPLOG. The action taken by the as/400 when this occures is in the system value QMAXSGNACN:

Display System Value

System value . . . . . : QMAXSGNACN
Description . . . . . : Action to take for failed signon attempts

Action to take for
failed password
attempts . . . . . . : 1=Disable device, 2=Disable profile, 3=Disable device and profile

You mentioned that AS400 screen just went on blank, i might get involve in two reasons
1) Multiple times of invalid sigon(max of 3 signons) will gets the userid gets disabled.
For that u have to see the userprf and have to change the status as *enabled.

2) if the screen went on blank for sometime of period, it might because of a backup might be performing(qinter sbs will gets in down state)

Any queries let me know

Thanks
Anand

In simple terms.
If you have a user profile name jsmith working at device ship02. User jsmith forgets his new password with 3 invalid attempts at device ship02. User jsmith can’t sign on anywhere and no one can sign-on at device ship02.

To correct this type wrkusrprf jsmith. Change the disabled status to enabled. This will take care of the user profile being disabled.
To correct the device being varied off. Type wrkcfgsts *dev. Navigate to the device ship02 and take option 1 to vary it back on.

DLM