+1 on the previous comments - make sure there is nothing stuck in the tire and poking through that may have repunctured the tire (thorns are a popular culprit around here). Also make sure there is nothing on the rim that may cause the flat, like a burr, spoke poking through the nipple head and rim tape, rim tape not covering all nipples, etc. If it lost 10 PSI or so, its probably natural air loss through the porous rubber, meaning probably no puncture. If you are losing a lot of air in a short period of time, like 1/2 the air pressure over 2 days, it could definitely be a small puncture with a slow leak.
A good trick for finding exactly what caused the flat is to take the wheel off the bike, and with a piece of chalk mark 1) the sidewall of the tire at the valve stem, 2) the sidwall of the rim at the valve stem, 3) the valve stem on the same side that you marked the rim and tire. Then take the tire off, take out the tube, and pump up the tube. If the tube is flat, air will be leaking out somewhere, feel for or hear for the leak. Once you find it, line the tube up with the rim and tire using the chalk markings and inspect the rim and tire in that area for something that could have caused the puncture.