cat hiding in the garage -- help reunite her!

Our cats, after 2+ happy years together, are at war -- and one's hiding in the garage!
Tootsie is 4 years old, a half-feral female cat who we adopted at 6 mos. She is a little skittish around strangers, but very loving to her humans (2 adults and an 8 year old boy.)
We got Buzz 2 years ago as a kitten -- he is larger than Tootsie by a few pounds, and very bold.
They've always wrestled (mostly Buzz pouncing, but Tootsie can hold her own, and runs off if not interested), but otherwise are very cuddly, sleeping together and cleaning each other.
....Until 10 days ago, when they had an all-out hissy fit fight in the middle of the night. Since then, Tootsie growls at Buzz on sight, and increasingly started hiding out in the garage. I don't think she's eating, drinking, or using the cat box (she's taken a spiteful poop on the rug, and made a few puddles here and there.) We've been able to coax her out of hiding for some petting and food, but she won't rejoin the household.
Both cats are neutered/spayed. Both are indoor cats (altho, about a month ago, we started taking Buzz out into the backyard on a leash for short excursions. We do not do so with Tootsie as she hates both the leash and the great outdoors.)
Any advise on how we might reunite our family?

I would first of all be sure both cats check out health wise. Sometimes when two cats who normally get along great suddenly start spitting and hissing at each other- it can mean that one cat is ill and his or her scent is "changing" and making him smell alien. That would be the first step-

I agree with Hissy a 100%

What also come to my mind , maybe Buzz picked up a scent from the outdoors what Tootsie hates . It could be a scent from a other cat what Buzz picked up ,out in the yard while walking on the leash . That could freak her out . But a vet check would be for me a # 1 thing to do , to rule out any health problems .

If the result of the vet check is that both kitties are well, you can try the vanilla extract trick. Place a dab of vanilla extract under both cats chins, between the shoulders and at the base of their tails, 3 times a day. This helps the cats to smell the same and mask any odd odour Buzz may have picked up in the garden.

Something else that has been found to be successful is a Comfort Zone plug-in. You can read about it here.

Good luck with your two cats.