Start by deciding whether the screen or the source is at fault. Connecting an external monitor answers this immediately: if the external picture is perfect, the panel or its cable is the problem, and if both are bad, the fault lies further back.
Flicker is usually a refresh or cable issue. A cable that cannot carry the resolution and refresh rate being sent will produce intermittent flicker, sparkles or brief black frames, especially with long or thin cables.
A blank screen with the machine clearly running is often a display-output problem. Laptops can send the picture to a monitor that is no longer connected, and the built-in panel stays dark until the output is switched back.
Distortion in patterns or coloured blocks points at the graphics hardware or its driver. If the same distortion appears before the system loads, the hardware is the cause; if it only appears afterwards, suspect the driver.