I need more information to properly answer your question.
What kind of orchid it it? Is it a Phalaenopsis? How long were the flowers on it?
Did they fall off quickly?
They may have just fallen off in the the natural order of things. Or perhaps the roots of your plant are in a media where they do not drain properly. If this is the case then more than one leaf will yellow.
Make sure the pot can drain and does not sit in water. This is the first thing to look at. IF it is sitting in water then get it into a pot that drains.
You many not be doing anything wrong.
As to the humidity issue you can put it in your bathroom if there is enough light for the kind of orchid that you have. Or you can put them on little trays with pebbles and add water to the pebbles. The orchid plant and pot should sit ABOVE the wat NOT IN IT. This will increase humidity right around the orchid. Check out my humidity tray page. http://www.orchids-plus-more.com/orchid-humidity-tray.html