As explained from the beginning, the stylophone is a very robust little instrument, and an apparently dead unit is usually something quite simple.

The single most likely cause is a dirty stylus and keyboard.
It can be very easy to overlook this, and suspect a failed component, or something just as drastic.
Over time, the keyboard will become tarnished - as will the stylus - and a simple clean of both works wonders, and often addresses the problem. Even if the stylophone is working, a bit of metal polish here can improve the sound quality 100%, compared to a dirty unit.

Troubleshooting

You put in a battery, switch on, and apply the stylus to the keyboard - nothing happens!

Depending on what model you have, the tuning control could be at fault.

On some models the rotary tone/pitch control is as shown here; basically with the wiper arm exposed..

There should be a small graphite peg located in a hole at the end of the arm. They are held in place under a small downward pressure from the spring-like arm. This arm can become fractured along its' fold, or simply become slack. If the tiny peg is not in contact with the black disk below it - or if it has fell out and missing altogether - the stylophone will not work at all!.

The stylus wire is probably what most people will suspect as the fault with a non-working unit, but in all honesty - unless it has come completely adrift - it is unlikely to be the cause.
The reason being that the wire passes through a small hole in the case, where a simple knot is tied before being soldered to the board. Even a sharp tug on the wire is unlikely to dislodge it. The other end of the wire is soldered and crimped into the stylus nib. The cracks that appear around the lead where it folds is usually cosmetic damage only. The lead itself is made up from multi-strand wire; (similar to that as used on speaker cone connections, which have to withstand constant movement); and is very resilient. It's worth a look of course, but to check it properly you will have to dismantle the stylophone. Going this far will let you get at the second most likely cause anyway - the sliding switches.

If you have gone through all the above, and the Stylophone still doesn't work, it could be that you just have one of the less reliable models; as explained on the 'Basics' page; and component failure is at fault.
Finding the problem is going to be one thing - obtaining the necessary parts is another!.
Some of the boards/circuits are going to be a real problem, and you may just be left with a 'display' stylo.

The Stylophone Sales Corner do have some genuine replacement parts for these various units, but they are usually kept in reserve, for their own stock of guaranteed reconditioned stylophones.

Just something to bear in mind if 'shopping around' for a used example!.