Sony CDP-CX400 CD Changer

How do you control it? Remote? Keyboard?

If it's a remote, are the batteries ok? If it's a keyboard, has any key stuck down (this will stop other keys working).

 

Disc stuck in the main player mechanism ....???
Maybe broken belt..??

 

Is the remote working? View the end of it using your mobile phone camera - you should see the IR LED working when you press any button(s).

Can you 'reset' the player by turning off at the mains and powering up again?

 

Drive Belts are done, practically erode into goo (really nasty tary gunk).
Amazon sells the replacements, you'll need 2, for about $12 per pair: Part X001PK6EIR
The surgery is in depth, complete disassembly of the unit required, a few dozen P2(regular philips) screws, about 2 hours. Acetone (nail-polish remover) for old belts removal/discard and cleaning of drive pulleys.
Aptitude Req'd: 5 of 10 (attention to disassembly essential, then reverse order)

Sadly in my case the CD reader was also deteriorated (skipping, dodging tracks) so restoring the turntable's motion/function was not the whole of the repair. Since the above required demounting the laser-side of the reader it's likely a better economy to address it's replacement in due process.
It's no wonder, though, that SONY no longer mfgs these units, as nifty as they are.
Good Luck!