monitors

I found this massive nec 20 inch flat screen but it only displays one thin line horizontally in the middle, can i fix that?

It depends where the fault lies.

If the fault is in the steering coils they will be expensive to replace and difficult to get the alignment right. However, there's little chance that this is where the fault lies unless they have been physically damaged by someone poking about in the back of the set.

That means that the fault is more likely to be in the electronics which controls the signal to these coils. The components which do this are now so common that they are very cheap. Without a circuit diagram you won't find it easy to work out what's what.

The only way is to look for burnt or damaged areas on the circuit boards, and for bad solder joints. If there are no obvious damaged components you could try simply resoldering any joints which look dodgy. All you actually need to do is remelt the existing solder, but you'll probably find this easier if you use a thin fluxed solder to help things along.

Start on the small PCB at the back of the tube. If you are lucky it could just cost you small amount of your time to get it up and running.

The vertical circuit is in need of repair. Unless you have experience in electronics and access to a schematic, I can guarantee that attempting to repair will be an exercise in futility.

my .02¢

getting new one is better