There are numerous factors to consider when staining or refinishing a floor or piece of furniture. Among the most important is humidity. The long drying time of polyurethane causes it to absorb a high level of moisture from the air. If moisture becomes trapped beneath the polyurethane it can cause it to "blush," or take on a cloudy white haze. There are solutions to counteract this problem, however. Some solutions are fairly quick and easy and some are less so, depending on the project.
Waiting is not the most immediate solution, but it is the easiest. As the humidity lessens, the drier air may allow the trapped moisture to evaporate on its own, leaving your surface clear and attractive. However, this works best in areas where humid weather is not the norm. In areas of consistent high humidity, you'll probably need to try another option.
If you live in a humid area or if waiting is not working, help the evaporation process by running a hair dryer over the blushed area. Don't get too close to the polyurethane as this could cause bubbles to form. This can take several minutes, and while it often does a good job at evaporating the moisture, it's not a practical solution if your project is a whole floor. If your floor only has a few spots of blush, or your project is smaller, hand drying is worth a try.
Applying a coat of lacquer can also help. Lacquer requires good ventilation. Lacquer dries much quicker than polyurethane--usually in minutes--and because of this has a less time to soak up moisture. If you hand dry the lacquer, it will take almost no time at all to dry and will not be able to absorb moisture at all. This is often more feasible with smaller projects, however, as you may have to lacquer the entire floor to get a consistent look--and it may not be the look you wanted.
If all else fails to clear up your blushing, or if you have such a large area of blushing that other methods prove inefficient, you may have to refinish the project entirely. If it comes to this there are some things you can do to prevent the same thing from happening again. The first is to do the work on a less humid day. If you are working indoors, use a dehumidifier to dry the air in the room or use your air conditioning system to circulate air from outside and create ventilation.