Ceiling Light Won't Turn Off After Replacement
Switch loops 101

What you have here is a classic example of an old-style switch loop.  The white wire capped off by itself is the switched-hot coming back from the dimmer in the wall, which is why the breaker tripped when you nutted it in with the other whites -- you had created a short circuit from switched-hot directly to neutral.  So, to get the fixture working again, with the power off:


Un-nut the fixture blacks from the box blacks -- leave the box blacks nutted from each other.
Nut the solo white wire to the fixture blacks.


And then you should be able to button everything back up, turn the power on, and at least be able to control the fixture in an on/off fashion.  You'll need to replace the dimmer as well, since it likely died (failed short circuit) during the earlier breaker-tripping mishap.