Those are all great suggestions -- I appreciate you taking the time to really consider it. Let give you some more detail about I did this last time to resolve the issue. When it happens, as it did in this case, the left channel just goes silent -- it doesn't just reduce volume in one channel like what my old HK795 did before it finally quit altogether a few years ago. This last time, the left channel did come back on a time or two briefly, as near as I could tell entirely at random.
Unless I am certain of the source, I typically make sure I've got both channels working at the computer first by unplugging the USB that goes to the DAC -- the audio then comes out the computer speakers and I can hear that it is working correctly. Plug the USB back in, the DAC gets identified and powered on, but the left channel is still out. You would expect that any short protection would be resolved with the powering on, but that doesn't appear to happen.
So I turn the amp off and back on. No difference. One thing I've not thought to do before is check if the amp is feeding both channels from the second source (the Topping amp has inputs for two sources). I did switch out the sources and it didn't effect the left channel issue, but I didn't power on the second source to see if both channels were working at the amp. I'll try that next time.
The next thing I did was plug some headphones into the headphone jack on the DAC, but all that did is reveal that the left channel was also out in the headphone circuit. Then I fiddled with the left channel RCA plug thinking it might be loose or something (for no reason since I never touch the DAC unless it is giving me a problem). The plug was well set, but I unplugged it anyway. I was handling the plug and touching it to various things (computer, amp, etc.) to see if I could hear any feedback of any kind. This may seem odd, but my old HK795 would give different feedback-like sounds depending on what an input plug was touched to. This was in the time before it finally quit -- it was sending a lot of noise into one channel for a while. There was no noise from the Topping amp, however. But when I did eventually plug the RCA cable back in, the left channel was working.
Here's the DAC:
https://www.amazon.com/gp/product/B01M7QQQC7
Here's my other DAC:
https://www.amazon.com/Dayton-Audio-DAC01-24-bit-Output/dp/B07JH8BKWN
Like I said, both have had the same issue. During this time, I have changed computers, but the amp has always been the Topping TP60. The cheapo DAC was actually my first one, but the channel dropout issue made me buy the Dayton Audio DAC to replace it. However, the Dayton Audio DAC did the same thing and it seemed to me, more frequently, so I switched back to the cheapo. There was another issue with the Dayton Audio DAC that I can't now recall, but it ended up in the garbage.
I wish I had access to another amp, but I don't. If this issue could be traced to the amp, I might consider getting something like the new Topping PA5 to replace it, but I don't yet see how the amp can be causing an issue like this.