Check power to transformer.
If you plug a light or power tool into the circuit does it come one?
Check power out from transformer.
Most transformers use a photo cell switch. Photo cell failure is the most common cause of big-box-store (home owner ready) transformer failure.
The photo cell will need be taped over with black tape. A photo cell is a light sensitive switch, and can take several minutes to switch on. If no power out look for fuse and or reset switch.
Transformers can also use a fuse and or a reset (circuit breaker) switch. Most common a reset switch.
A photo cell is wired in series with the line voltage, and on some the secondary low voltage side. Many of the less expensive transformers come with proprietary connections, where purchasing a replacement that plugs in is available. If you can find it. This is why we we use commercial transformers and wire our own. Saved time and is easier in the long run. We also get reliable service from this.
You can open any transformer and measure where voltage is or is not; and debug the circuit. All most never will the coils (the heavy part you can't fix) be the problem. In wet conditions with older installs, you may be better off buying a new transformer. Simple voltage checks will show cause, if not replace the whole thing.
The transformer is simple to hardwire or install a cord. When we install cords, we buy low cost extention cord in whatever length, usually 6 to 12 feet. Or, hardwire with #14 UF. The nice thing about having to wire it, is you now have ability to mount the transformer in a reasonable location out of weather as possible, as opposed to constraint of a shorter cord.
Got low voltage output? (about 12 volts)
When power out of transformer is verified, check for voltage at the nearest light. Keep in mind bulbs and fixtures fail. Most common problem is poor wire connections
About Wire Connections
The quick connectors which accompany most outdoor low voltage fixtures are designed such that moisture gets in the wire. First thing we do on a repair is remove the connectors, strip the wire back 1 1/2 inches. Then twist a solid connection and tape it well. You will be surprised how a really good connection can affect bulb life.
Wire Connections Make the Difference
Landscape light wire sits in moisture. Quick connects do not last. The connection area receives moisture, connection becomes poor. The new mechanical wire strippers allow the following connection be easy and fast. If it seems like over kill; at least you know your connections are strong and will last.
Cutting off the quick connector, strip twist, and tape. Best to strip at least 1 1/2 inch. We twist the wires as much as possible. Tape real well. The connection doesn't fail, you can see improved bulb life. Poor connections do cause early bulb failure. Bottom line is the more water gets in the wire, system will fail faster.
New style wire stripper makes quick work of this.
Self-Adjusting Wire Stripper
Your low voltage landscape wire lays in the ground, where moisture collects.
The above connection sequence is not perfectly moisture proof; it is far better than the quick connects; also many times better connection that will hold up over time.