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.
You put in a battery, switch on, and apply the stylus to the keyboard - nothing happens!
Another place to look is the battery clip.
The switches themselves can also be the cause. On most* of the models, they connect directly to the circuit board - much the same way as applying the stylus.
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.
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.
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!.