looks like 24v for me

AC is ok, they don't quite care.

So after measuring at the output, doesn't have 24Vdc?
just happen lately or since day1?

For AC --> DC converter normally rectifier gg
this is the typical sch (ignore the values) https://www.circuits-diy.com/wp-content/upl...m-Schematic.png

not expert, just co-curriculum.
Rectifier == 1N4007 from the picture.

OptionA:
If this adapter is cheap, i would suggest you buy new cause not worth the risk to play with 240V. You might cook yourself before you fix it.
Just goto any hardware shop and bring your end device with you if possible to test up.

If your end device is huge, just bring the adapter, they will able to find one for you.

OptionB:
If DIY is truly your route, from your description of <either no value or not a stable value> then yea, most likely is the rectifier, you just need to change that.
Normally it is something has 4 pins --> mind to take a picture of the boards (top and bottoms)?
Typically looks like this


internally:



you just need to measure it (without power on) using the diode sign on your multimeter (assuming using digital)


long story short refer to this

if you look at the initial thread, you will find a transformer there and i suspect the one IC beside the heatsink is the rectifier but without proper image, i cannot confirm