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This is a presumption, but my guess is that you have lost interest because there is no drama in your outline. The peril of outlining is that causes you to focus on events. Events are not dramatic in themselves. There is no drama in a set of turn by turn directions, for instance. They will get you to your intended destination, but unless something exciting or interesting or disastrous happens along the way, it is not going to make for great dinner-table conversation. 

And if your outline is indeed just a set of turn-by-turn directions to get your plot to its intended destination, you are not likely to feel much like writing it down, because it is all there already. There is no drama, and therefore nothing to get interested in writing. 

Plots exist to create drama, to force the protagonist into situations where they have to make tough choices. If you plotting gets ahead of your sense of drama, it is going to create an outline that is not interesting enough to expand into a story. 

If you are at the 50% mark, you should (by the numbers) be right at the heart of the drama. Are you? If not, the problem probably does not lie in your outline for the second half but in your written-out first half. I don't think it is terribly unusual for writers to get half-way through a novel and realize that it lacks drama. If the drama is not palpable at the mid point, then there is something wrong in the first half and writing the second half is not likely to be appealing. 

So, look for the drama. Look for the hard life-altering decisions you hero has to make. Find the drama and the desire to write should return.