Nothing, unless you're allergic to it (which is very uncommon.) Zoloft is metabolized very slowly; it would take at least three weeks of a steady daily dose for any of the effects to start. Even when they do, it's a very mild antidepressant, and one hundred milligrams is a modest dose.
If you took it by accident, don't worry unless you start having strange symptoms. If it was prescribed to you, I'd give it a try. You may not realize you're depressed, but if you are when the zoloft starts working you may notice a pleasant change.
I've been on a variety of anti-depressants, including this one. Even though I made the decision to stop taking psychoactive drugs (with the exception of coffee,) they did help me when I needed it. The people in the medical field know what they're doing: it's a good idea to listen to them. It never hurts to get a second opinion though.