This has happened to me more than once. All of a sudden, the main character loses all touch with reality, becomes boring, and wants to be a hermit, refusing to talk to any other characters. If you're not a writer, you'll think I'm crazy or something. But writers understand this. You're writing along and WHAM...your character falls flat.
What do you do?
If that character is falling flat or seems to be that way, that means another character might be trying to steal the limelight (it happens a lot). Figure out which character that is, and have your flat character take them on. Fight it out. First, it wakes up your flat character and makes them fight for what they want (their motivation in the book), and second, it puts the loud character back in its place--the role of minor character or even antagonist in the book. It makes for conflict--what books are best at.
Have a great week!