"Check signal cable" and similar messages occur when a monitor detects a connected cable but cannot detect a device or video card on the other end of that cable. This usually occurs when a computer is powered down or a video card is not correctly installed.
To resolve the issue, make sure the computer is powered on, then disconnect and reconnect the cable. If the message persists, shut off the computer and check to make sure the video card is seated correctly. If the monitor allows input from multiple sources, make sure the proper source is selected in the configuration menu.