Whether you know it or not, your dog probably loses a few whiskers every so often. They grow back; it's usually not a big deal. Visit a vet if you're worried.
Not Just Hairs
Your dog's whiskers aren't just longer, coarser versions of the other hairs on her body. Whiskers -- technically vibrassae -- are special sensory hairs that are rooted deeper than the rest of her coat. When something brushes against them, say, your hand, a wall or breeze, they vibrate, sending a sense signal to your dog's brain. This allows her to gauge distances better, particularly at night and at close range. Whiskers give your dog a sense of space, ergo their loss reduces the amount of information she has about her surroundings.
When They're Gone
Whiskers are like radar: They provide a map of the nearby world. It's natural for them to break, splinter or fall out from time to time, but it's usually just a few at once. Whether through trauma or disease, your dog may lose a bunch though. This confuses your dog, who now lacks the ability to gauge close distances. She may seem off kilter or a bit clumsy. Whiskers don't exactly affect dogs' sense of balance -- as with most vertebrates, that's controlled by the inner ear -- but it does help orient them. Expect your whisker-less dog to seem a little less like herself. She may even act afraid or suspicious for a spell.
What To Do
In most cases, whiskers grow back on their own. That's not to say it's a pleasant process, only that the loss is temporary. That is, of course, assuming you address the underlying cause of why your dog lost them. There are few medical conditions, if any, that only affect dogs' whiskers. Most causes of alopecia -- the technical term for hair loss -- also affect the rest of their coats. A vet can help diagnose the problem and prescribe treatment, be it pills or shampoos. In the interim your dog may continue acting frightened or clumsy. As her whiskers regrow, she should gradually get back to her old self.
What Not to Do
Never trim your dog's whiskers. Every time you do that, it changes her orientation with the rest of the world, which can be quite traumatic. As if that weren't enough, there are no non-aesthetic reasons to trim them in the first place. If you take your dog to a groomer, make sure the groomer knows your preferences about this -- despite the potential affects on your dog, whisker trimming is still a fairly common practice.