As a primarily self/internet taught electronics repair person, here would be how I would approach this. Assuming you are a beginner to this and don't have a lot of tech toys to troubleshoot this we will only use a multimeter (available for $5-$70 depending on how many bells and whistles you want, this one for $6.40 on amazonshould work perfectly fine). for a description on why you want a multimeter/why it is useful, see footnote 1.
I will also assume that we don't have a schematic of the device in question because lets face it, most of the time you don't.
Lets take your scenario, in which a customer (friend) tells you their device (a smartphone for example) doesn't turn on.
first try to ask them what they think happened. If this is a device that doesn't turn on randomly, or if it was acting odd (i.e. erratic performance, not attributable to a single point of failure) before it stopped working, then odds are it is water damage, (to treat this look at footnote 2.)
If it doesnt sound like water damage and you don't see any when you open it up, then start double checking the device yourself. The rule of thumb when repairing something that isn't yours is: Unless you saw it happen, what they said happened is just their perspective, and should be checked again by you. (there are so many times that I haven't double checked what a customer said and that turned out to be what was wrong that it should be a rule.)
Lets go over what we know:
It doesn't turn on, (also, no signs of water damage)
since electronics run on electricity (I know this is rudimentary, but bare with me) trace the electricity.
all electronics have a closed circuit, so you can follow the circuit and check to see where there is a connection.
If it has an internal power source, check the battery with your multimeter. What should the voltage be at? (google nominal voltage) If it is charged (as determined by voltage readouts), trace farther up. If it doesn't have power, check the charger, if yes then plug it in and check the battery, does the battery get a charge? if no then there is a problem is between the battery and the charger, check intermediary points on the connection when the usb cable is plugged in, as soon as you find a point where the voltage picks up again, you have just narrowed your search. if this is a circuit board, check for any cracks in the solder that could be causing a faulty connection.
Note, just because you find and fix one problem, that doesn't mean it will suddenly work again, there could be a few problems with a device, normally a root cause which in some cases caused a problem, which caused another problem, which caused another problem... you get the idea.
I hope this helps you further your technical fixing skills!
1. A Multimeter is useful for many diagnostic reasons, but for the basics, you care about four things.
a: Whether two points are connected (by checking resistance, if the loop closes it will display the resistance on the loop, if there is no resistance it will show a 0 or beep. If there is no connection the screen will also tell you that. This is useful because it helps you trace your way through a circuit board via trial and error). This is also how you would check if a resistor is blown.
b: the voltage between two points, (allows you to check battery charge levels, as well as voltage drops in a circuit.) (in case you are new to electricity, a basic analogy of voltage is the pressure of water in a pipe)
c: The amperage between two points (going back to the water analogy, Amperage is equivalent the flow rate of water), this is helpful for determining the amount of drain being experienced by a power source i.e. battery).
2 if a device is exhibiting symptoms of water damage the first step should be opening the device up to just double check that there isn't water still in it, shorting something out. Open it up, if water is there disconnect all power sources (including battery if possible), next remove the water, (an optional step here is to pour 97% isopropyl alcohol in the device (but try not to pour on a screen, it can mess with adhesives) the benefit of this is the alcohol will often displace the water, meaning you now have a device that is now soaked in alcohol, still not ideal but way better because alcohol evaporates faster than water and is less corrosive)this can be accomplished by drying with a hair dryer (not on high), next put in a bag with some dessicants (those little bags in food that say do not eat, can be purchased on amazon for cheap) for a day or two, reassemble and voila!