Your initial symptoms followed by a loss of picture altogether leads me first to perhaps looking for an arcing issue. Usually sync instability can be traced to an ungrounded external conductive coating (aquadag) on the kine, such making it impossible for the tube to act as a final filter capacitor for the HT. This can lead to horizontal sweep frequency spikes electrostatically radiating from the ungrounded coating and interfering with circuitry that should count on such sync pulses from the received signal only to keep the sweep in sync. This conditon will obviously lead to trouble.
Now it is also possible that you have instability issues due to a defective horizontal hold pot if that is what is used, or a bad horizontal oscillator coil, or a poor solder connection. Such may have been intermittant at first to cause your instability issue, followed by out and out failure of the connection altogether....
Can you hear the 15.75KHz whistle as it should sound normally. (I did as a kid, and could diagnose a horizontal sweep/ HV problem in a heartbeat just by the volume of the whistle--but as an older fart no longer can as readily.
If you can then that is to your advantage--if you know what it
should sound like for that set. You can also take an NE-2 neon bulb test light and ground the leads, and hold it close to the HV lead. It will readily glow if HV is present.
If the latter-mentioned condition be closer to what you are looking at you will need to do some deep sleuth work, and check over the horizontal drive circuitry with a fine-toothed comb. Beware in the troubleshooting process that if you are operating your set and there is no horizontal sweep signal to the output tube then that tube can cook itself to death in short order owing to lack of drive derived negative bias on its control grid. Keep an eye on its plate for an increasing orange glow.