Could be a problem with the computer, cable or monitor.
There are two push button switches at the rear of the 1084S monitor which need to be set to the correct position to select RGB input, check these are correct.
These monitors have a common fault where the 9 way D-range socket develops cracked/dry solder joints.
Try wiggling the connector and see if you can get a picture. You may need to have it resoldered.
