I am a bigtime electronics nerd, and I can shamelessly spend way too much time digging into things that other people would deem silly. I found myself having a very weird behaving 2x49W T5 HO fixture recently. It uses a Philips HF Performer, HF-P 249 TL5 220-240. It works absolutely flawlessly when it's warm, but as soon as the temperature drops, it refuses to start. Not even a blink, or tube glowage, nothing. It does not even draw any wattage.

My obvious suspect is failing capacitors. So I took it apart, but to my surprise, the majority of capacitors are actually polyfilm, which more or less never go bad. There is just two electrolytics in the whole thing, the main filter cap for the rectified mains, and then a small one presumebly for filtering the supply going to the PFC chip. I yanked the small 47uF 35V cap which acts as a filter for the PFC chip to measure it, but it appears absolutely fine, so I will likely put it back. Now my suspect leans more towards a transistor or FET or something like that on the thing which may not like the cold. I plan on troubleshooting it more tomorrow after work, I think I will put it back together, and try to start it in the cold again. Assuming that it still does it, which I think it most certainly will, I think i will take my heatgun and heat around a little and see if we can pinpoint exactly what is wrong!

I just wanted to share a little about this, I am curious if there is anyone else out there who has repaired ballasts before, and I am also curious if anyone else has had a similar failure mode! this is definetely a peculiar failure!

At first I would thought maybe the tube was having cold temperature striking issues, but even that would probably draw some measurable current from the mains. Maybe it has some sort of protection circuit that turns off the ballast if the tube doesn't strike? What kind of temperatures are we talking about here in terms of cold and warm?

Likely to be the main electrolytic capacitor that's gone, I had this with a Tridonic 18W TC-D ballast, it worked perfectly when warm but refused to do anything when cold. When I held the ballast PCB over a heater for a few moments and then tried it, it came back to life immediately.
The EL cap did run noticeably hot when in use, but replacing it for a suitable equivalent restored the ballast to normal again.

There is a lot about the ESR of the caps and this changing when they dry out, and of course you ned to get a replacement one of the correct values, though at the moment a lot of that is above my understanding of the subject.
I do also wonder if this may become an issue with old NOS HF ballasts that have been stored for a long time, if the caps eventually dry out.
Hopefully your ballast should be repairable though as it has failed like this rather than with a "bang" which usually means something more serious!