If there’s no apparent legitimate cause, I usually wallow for a little while, until I get sick of it.
But I am not trying to “bring the happy back” or put a smile on my face. I’m just trying to feel less lousy. And that usually happens best when I stop thinking about it.
Sometimes doing something constructive is the answer. Sometimes escapism is the right potion: music, a movie, a book, some web surfing. Sometimes the best thing is just to do something that takes me out of myself, like fulfilling a commitment to help someone else. Sometimes we just wants a nap.
Sometimes making my husband listen to me complain and getting sympathy (which he’s been coached over 30+ years to give and now does fairly decently; men are trainable after all) does the trick, and I feel fine after that.
It usually doesn’t last too long, in any case. I’ve learned that even the worst day isn’t terrible every minute, and I’ve been working on putting my attention where it’s most beneficial.