It could be the dimmer switch. The only way to know is grab your test light and start checking things out. I am certain it isn't a 1 bulb failure taking out the rest. I have had single bulbs quit and the other 3 kept on working.
Remove the dimmer switch and check for 12 volts going in and 12 going out when turned on to max. If you have no power in then trace the wiring back to the fuse panel, either you missed a bad fuse or a wire is broke or has a bad connection. Don't forget to check the ground or negative side of the circuit, no ground = no electricity flow. If you have power in and none out then it's probably a bad switch if your ground is good. Yes it can fail all of a sudden like that.
If you have power out of the switch then check for power at each individual light terminal. No power means a bad wire or connection between the switch and light. Also make sure the ground is good to the light socket. Having power to the socket and a good ground means a bad bulb. I think I remember paying about $8 for 2 of those halogen bulbs and having a hard time finding the right wattage so my replacements were a bit dimmer than the originals. They don't last as long as you think they should. I believe I had replaced all mine within the first 2 years but they didn't all fail at once like yours which makes me think it isn't the bulbs on yours. You should still have some spares on hand for when they do fail. Good luck.
