Regardless of when she gets to descend, and whether she's governed by the approach ban or visibility, or MDA/DH alone, a pilot who fails to obtain the required visual reference to land must conduct a missed approach, and go somewhere else. If conditions are rapidly improving, or she has some other reason to believe that it's worth the fuel and time to make another attempt to land at the same airport, she may return for a second approach, but typically she turns for her alternate.
In Canada, every IFR flight must depart knowing that the weather at a specified alternate destination is forecast to be better than needed for landing. Not just good enough, but better, and they do tell you exactly how much better, depending on what sort of instrument approaches serve the airport. The weather is quoted in terms of ceiling -- the altitude above ground level of the bottom of the lowest layer of clouds, not counting layers that cumulatively do not cover half the sky -- and visibility -- how far an observer or RVR can see along the ground. These numbers are compared to the decision height or minimum descent altitude. MDA and DH are quoted in feet above sea level, because that's what the pilot reads on the altimeter, but this calculation requires feet above ground level. This is called height above touchdown, or HAT. For approaches where it may be necessary to turn or overfly the runway and circle back in order to land, the slightly different height above aerodrome elevation, or HAA applies.
If the alternate airport has two or more usable precision approaches to separate runways (not just opposite ends of the same runway), then for the typical ILS approach, the ceiling must be at least 400 feet and the visibility at least one mile. The typical ILS approach has a decision height of 200 feet agl and an advisory visibility of 1/2 mile. If, for some reason, the published DH or advisory visibility is greater, then in order to file that airport as your alternate, the ceiling must be 200' greater than the HAT and the visibility must be half a mile greater than the advisory visibility. We call this weather requirement "four hundred and one" and write it as 400-1.
If the alternate airport only has precision approaches serving one runway, then the weather needs to be even better. I'm not entirely sure of the rationale for this, because while it definitely gives you a better safety margin to have an extra runway available if something happens to one of them, even perfect VFR weather isn't going to improve your chances of successfully landing at an airport whose only runway has been closed due to a massive fuel spill. But when there is only one usable precision approach, the minima are 600-2, meaning a ceiling of 600' agl, which must be at least three hundred feet above the lowest usable HAT, and a visibility of at least two miles, being at least one mile greater than the advisory visibility.
Should the alternate airport be equipped with non-precision approaches only, the weather minima change to 800-2. The ceiling thus must be at least 800' or 300 feet above the lowest usable HAT/HAA, and the visibility at least two miles, still at least one mile above the advisory vis.
That's just the intro to Canadian alternate minima, but that's enough for one day. Tomorrow I'll tell you what happens if there is no IFR approach or only a GPS approach, and show you how to compare round number ceiling forecasts to the very specific numbers on approach plates. Also, see if the Americans who don't know can guess what it means that the standard alternate minima can slide.