Before troubleshooting the engine/train itself you need to be sure that your transformer is working correctly. You can find an inexpensive voltmeter (or often free with a coupon) from Harbor Freight stores. You need to have a basic understanding of electricity (and it goes without saying safety) to work on electrical equipment.
If you do not want to troubleshoot anything, you could try to replace either the locomotive or the transformer. The likely point of failure is the the transformer, though it could be the engine. You can find a lot of used equipment on ebay. Or you might find a local hobby shop that might could connect you with a local train hobbyist group that probably can get you up and running again. Good luck! And let us know what you find out!