//c 9" monochrome A2M4090 tilt / rotation

Very often such effect can be viewed if device with strong magnets ( like strong HiFi speaker, Bassbooster or very strong power switching supply or powertransformer ) is positioned close to a monitor.....

If the entire image appears rotated, it means the deflection yoke around the neck of the CRT has moved.

Easy check: fiddle with the brightness and contrast until you can see the background raster. Are all the horizontal lines tilted the same amount? Then yes, it's a rotated yoke.

It's a simple fix: remove the monitor's back case (I've never opened up this particular monitor, so I can't offer detailed instructions). The yoke is the assembly of copper windings around the neck of the CRT. You may see magnets around the periphery; try not to touch or move these.

The yoke is typically clamped onto the neck of the CRT with a screw-tightened metal clamp. It should be pretty obvious.

Unplug the monitor and let it sit for 12 hours or so to reduce the chance of getting shocked. The CRT is like a big capacitor. Stay away from the mushroom-shaped rubber cover on the CRT with a thick wire leading to the flyback transformer. That's where all the high voltage is.

Loosen the screw, rotate the yoke, then tighten the screw again-- but not too tight, since you can crack or crush the neck of the CRT. If you feel brave, loosen the screw, turn the monitor on, then rotate the yoke by hand until the image looks right. The yoke looks scary but there are no horrible voltages on it. When you have the image straight, unplug the monitor and carefully tighten the screw.