Before we attempt to locate the cause of your problem we must first offer a warning: LIVE ELECTRICAL CIRCUITS CAN KILL. So, prior to working on wiring in your home, learn how to test for voltage, and don't take it for granted that testing one pair of wires in a box means that all the wires are safe. It is not uncommon to have power from more than one source in the same box. Application of Murphy's law here is a must. A book that will help greatly is,
Wiring Simplified, by H.P. Richter and W.C. Schwan.
You'll have to forgive this long-winded preamble, but your question touches on an extremely important subject matter, and deserves special attention. In our remodeling business, it is not uncommon for us to expose the contents of a wall and find where improper alterations have been made to the existing electrical system. Most prevalent are connections that have been accomplished by twisting wires together and then wrapping them in electrical tape, leaving the taped connection exposed to the wall cavity. Nothing could be more dangerous.
The major cause of electrical failure (save bad bulbs, switches, plugs and fuses) is a bad wire connection. If such a connection is made in an area where accessing it later necessitates tearing up walls or ceilings, the most common and simple of electrical repairs would prove to be very expensive.
The real problem here is the danger of arcing. A special, heavily insulated wire connection device known as a wire nut should be used to permanently attach groups of wires and to prevent damage from hazardous arcing, something that electrical tape alone has proved unsatisfactory in suppressing. And, when the spark does occurs it makes exceptionally good sense for the leads in question to be enclosed in a government-approved fireproof junction box. As a matter of fact the National Electric Code requires proper connectors, with all connections to be made inside an approved and properly sized readily accessible junction box. Taped connections made in wall cavities are unsafe and against the law. They're a fire hazard!
We warned you it would take a while getting to your answer, and here it is. Since your home is so new, the chance is extremely remote that an improper connection exists in a wall cavity. We have addressed the issue of bad connections, because that's what we suspect your problem stems from. Wire nuts can be installed too tightly by an overzealous electrician and this can actually start a break in a wire that vibrates loose over time. The broken connection may not be in the box where the light switch is located. It may be in another box between that box and the panel. To check (after power is off) start at the light box in question, remove the wire nut, and look for broken wire ends. Repeat the procedure from box to box, removing and replacing wire nuts until the bad connection is found.
A much less common cause, but a possibility none-the-less, happens after the electrician has completed his work. Occasionally, during construction, nails used to install cabinets, wall decorations, etc. can be pounded into electrical wiring. Usually, a short occurs immediately and the easy-to-identify problem is resolved under warranty. But we have seen occasions when the culprit nail only nicked one wire and, caused an open (no power) circuit. Tracing a wire broken inside a wall is less expensive to find than repair.
A simple continuity test with a buzzer at the other end of the wire in another junction box is one technique that is used. An electrician can usually find the problem in an hour or so. Depending on you patience, you may opt to do it yourself.
