This used to happen to me about every 2 days or so. The one thing that would fix it was rebooting the watch; just press and hold the crown and side button until I saw the Apple logo then wait for it to reboot. Then, a few hours later, the sunrise/sunset complication would work.
However, upon digging deeper, it seems that the problem is the watch periodically loses its internal GPS position and needs to update it. The way you can find out is if you switch to the Astronomy watch face, and look for the green dot that shows your position on the earth. If it's missing, it's lost your position.
The only way I've been able to get it to come back is to open the Maps application on the watch, and wait for it to find my location. Alternatively I can bring up the Maps Glance page and wait for it to find me. Once it finds me, the green dot shows up on the Astronomy face and then the sunrise/sunset complication will show a time again.
In the past, I removed the Maps glance and hardly every opened it up on my watch; I think that's what caused it to "forget" my location. So the past 5 days, I've been opening up the Maps glance once a day and I have not yet had a single day when the sunrise/sunset complication stopped working.
Hopefully this trend will continue!