Have you received proper training proper safety training, especially in the control of hazardous energy (commonly called Lock Out Tag Out)? Step zero of troubleshooting is to assess the safety of what you're about to do and make sure all hazardous energy is isolated/controlled before you go sticking bits of your body into machinery that can remove said bits (it's more complicated than that in practice). Troubleshooting is often an infrequent or new task, and infrequent or new tasks have a much higher rate of incidents, so you need to be very aware of safety risks when troubleshooting.
Once you have completed your safety assessment, step one of trouble shooting is understanding how the thing is supposed to work, both in terms of the desired external functioning (the stuff apparent to an operator using the machine) and also the internal operations that make all of that happen. This could involve talking to operators, reviewing manuals, drawings/schematics, etc. Next identify how it's deviating from how it's supposed to work. There is usually a fairly obvious way it's deviating from ideal operation, but that may or may not tell you very much, so you'll have to keep digging deeper. It's kind of hard to give advice for something that's so general, but I tend to like starting at one end of a chain of mechanisms and work to the other in a methodical way; don't just go randomly checking things willy nilly (outside of maybe a quick initial check of the "stupid stuff"). For example, if you have a hydraulic system consisting of a pump, some valves and an actuator, start at either the pump or actuator and work towards the other one in a very methodical way, checking each component as you go. Don't forget to take a ton of pictures and write stuff down. If in doubt, more pictures and notes.
