There is a fault in the Vertical Deflection. Coming good as it did may indicate a bad solder joint, although other causes such as a failing Vertical Output IC can cause the same symptom. If you value the monitor, take it to service now and the repair may not be too costly. If it is caused by something like a failing solder joint, the fault can recur, and if you persist in trying to use it in that condition, damage components, which makes the repair more costly.