if it is displaying a thin vert line ( top to bottom ) there is a problem in the Hsweep .
this is likely a computer type monitor and as such , they do have seperate supplies for HV and Hdeflection.
first step will be a visual inspection of the pcb . there will likely be a cold solder joint in deflection return, and
may be obvious with carbon tracks from arcing . clean and resolder , repair traces if needed
with any luck , there will be nothing further amiss....