What to search for
When we need repair documentation, we would prefer it to correspond directly to the make and model we are attempting to repair. (Sometimes there are "families" of products, where we can settle for a similar product and learn from repair information for that product.)
Locate the make and model number on the body of your e-thing. These are sometimes actually surprisingly hard to come by! In this case, you can start with a generic image search, entering the brand, type of device and any other identifying features into a search engine with image search. From an image gallery, you might be able to identify your specific make and model.
Ideally we are looking for more than one guide or resource, so that we can compare and evaluate the relative effectiveness of each.
In searching for repair information online, the "holy grail" is a manufacturer's own service manual. These often offer schematics, disassembly guides, and common repair advice, all in one place.
Some manufacturers have these on their websites (and they can be hard to find, even so!), but other manufacturers deliberately try and limit our access to them. Using a search engine, we can sometimes locate leaked versions of service manuals. There is even a black market for these.
Generally speaking, service manuals are easier to come by for older equipment that has a cult following, or a large hobbyist user-base, for example hifi and audio equipment.
If we cannot find a service manual, the next thing we are usually looking for is reliable information about how to disassemble a device, and therefore how it is put together and what it has inside. Disassembly videos abound on Youtube. The problem with these videos is that they are often very long, and often we are only looking for a couple of key moments in the video. In low-bandwidth or time-sensitive situations, they can be a nightmare.
So generally speaking, we would give priority to disassembly guides with still photos and explanatory text, in the style of iFixit. But other people offer them, like this firm that repairs hair straighteners.
Some parts sellers include repair information - and simple tools - when they sell a spare part. This can be useful but, as with any third part resource, these should be evaluated carefully and critically.
Threads on forums
There are a number of reliable, specialist forums for information on electrical and electronics repair and maintenance.
Sometimes these can be very intimidating, technical conversations. However if you are able to find a thread with detail about your particular fault - it could be the key to helping a Restarter save your e-thing.