Everyone who has ever done any painting has experienced it: you have no idea what you're looking at anymore. Does your painting still make any sense or is it a total failure. Especially after a long painting session you loose perspective of what you're doing.
Most painters have a trick or two to overcome these situations. One of them is asking a person whose opinion you value to comment on the painting. Looking at your work through somebody else's eyes can be quite refreshing, though not always a pleasure. Still, honesty is required if you want the critique to be of any value. I'm lucky to have a few honest people close at hand.
Another way to get a fresh view is to take a picture of your painting, open it in Photoshop (or similar software) and fool around with it for a bit. What happens if you give it more contrast or less light? If you know your way around this kind of software, you can change the color of certain areas and see what happens.
A much simpler trick is the mirror. When you look at your painting in a mirror you almost always see stuff that you missed before. I don't know exactly why. I think it has to do with the fact that the mirrored image is relatively fresh, but it's also easier to see your painting as a whole, instead of as a bunch of inconsistent details. Whatever the reason, it works, at least it does for me.
Oh, yes, I almost forgot the easiest of them all: take a break...