When the address will not load
Work top to bottom. Most problems are the first two.
Endless spinning, then a hidden service error
Your circuit is the usual suspect. Reload. If it fails again, request a new circuit from the Tor Browser and try once more. Then move to the second address. An address that refuses to load for one person and loads fine for everyone else is a routing problem, not a dead market.
Two of the three load, one does not
That is normal. The three strings travel through different parts of the network, and rotations retire one at a time. Use the two that work. Draw no conclusions from the one that does not.
None of the three load
Then the problem is between you and the network, not the market. Check that the Tor Browser is current, and that your system clock is right, because Tor refuses to work with a badly wrong clock. Then find out whether your network blocks Tor at all. If it does, the fix is bridges: Tor Browser settings, Expert, then enable a bridge. Bridge lists ship with the Tor Browser itself, so you are not taking the list from a stranger.
The page loads but looks wrong
Blank page, broken layout, a login form in a place you do not recognize. Reload once, then check the address bar against the block above. If the bar is right and the page is still wrong, try another of the three strings. If the bar is not right, you have a verification problem, and the verify page is the next stop.
It worked yesterday
Two causes, in order of likelihood: the address rotated, or your network changed. For the first, check this site and the market's official channels. For the second, the bridges step above.
A different .onion loaded than the one I typed
Stop there. Do not log in. See the verify page.
This site does not probe the addresses and publishes no uptime figures, so it cannot tell you whether a string is up at this moment. What it can tell you is that all three are the current ones, and that if none of them load, the fault sits on your side of the circuit.