Try uninstalling the game data for Mass Effect 3 and any DLC you do have (but keep your game save data). Then re-install them and try again to see if you can get the game going. I had a similar problem where ME3 gave me an error message saying that DLCs like Leviathan and Citadel weren't installed (although they were) even though the game worked fine the day before. Deleting the game data and installing the DLC again fixed that for me.
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