Visit an engineer or get a new one

LCD or CRT monitor? Anyway... You have a fault called intermittent frame collapse. The circuitry that controls the vertical displacementof the electron beam in knackered. You can probably buy a new flat-panel monitor for less than it would cost to fix (even though it is no more complicated than switching a board). The typical life expectancy of CRT monitors is about 5 years depending on usage, so even if you get it fixed, it probably won't last that much longer. If you have a friend who knows his way around analog circuit boards, he might be able to fix it if it's nothing worse than a bad solder joint.

your frames collapsed maybe just needs calibrating, but if its a crt dont bother yourself or youll kill yourself ,the psu takes your maons and multiplies it x100 get fingers too close to and 25kv eht will jump at you and kill you

same happened to me some years back,,,,,,, i got it repaired by experts

And i temporarily repaired it myself just a punc at back and its working temporarily!

9ice qppost