This has been known to happen, and electric dog fences are not a magic bullet. They require that you keep them in good shape and follow the manufacturer's guidelines. This means checking that you haven't damaged or broken any of the wires for your in-ground/underground fence by accident or because it is very old. Choose a strong gauge wire, and ensure you lay it out properly.
Your pup's collar may also have run out of battery, meaning you either need to replace it or recharge it depending on what fence you have. Last, but not least, check that everything which should be plugged in is plugged in. Sounds silly, but it wouldn't be the first time a vacuum cleaner wire or similar has loosened the connection to the wall!