You must first try to attack things logically. Was there any recent work done? Such as, maybe hanging a picture (poking a nail through a wire), any remodeling, any signs of cracks or water. Do you smell anything burnt? Many electrical things have a distinct smell, and that can lead you to the problem. Visually examine any of the wiring at say a box or fixture for evidence of arcing or heat damage, or contacts making a connection where they shouldn't be.
If those don't work to find the problem, you have to 'break' the chain from the source through all of the subsequent boxes until you can isolate the point where things work, to where they fail when you add those things downstream. Now, tracing the path of all of the wiring can be problematic, but there are devices you can buy to help with that...plug or attach a transmitter, then use a receiver to find where the wire goes. Proper use of one of those can guide you along the wiring's path. Worst case, once you've isolated the errant branch, you could run a new lead to it, bypassing the current section that has a problem.