Hi Meyrick
Taking it in stages: Putting aside your Classical Magic file issue for now. Let's diagnose in stages.
You say you haven't been able to use your SX900 for almost a month. Has the whole keyboard failed except the screen. Is it not responsive when you play it, or maybe you can't play it at all?
Have you Soft Reset your keyboard? I know it's not what you probably want to do, but it could well solve your problems. They are highly technical, and they DO glitch.
Try holding down the last white key on your keyboard whilst powering on your SX900 HOLD THE POWER BUTTON UNTIL THE MAIN SCREEN APPEARS then let go.
Now check if your files are being read... Yes/No.
You say you have your files backed up on USB'.
Have you got them backed up on your computer.
The SX900 has 2 USB ports, are you using both of them, are you reading from the correct one. Try just using USB001 on its own for now... Does that work? Are you using a USB hub?
If none of your USB'S work on your keyboard then as a test, format a brand new one and load say, a few styles onto it from this website ... Can the keyboard read them? Try usb 001 first, then usb 002.. What result do you get?
USB sticks are only for day to day use and storage.
Whilst accepting that it's highly unlikely that they would all fail, they often do. . Do you have the master copies of your files on PC. Preferably backed up on hard drive. Reload them, try again.
Where are your expansion packs stored? Are you talking about the default ones that came on the SX when new, or some you have downloaded? Have you exceeded the storage capacity of the keyboard?
Contact BeeSoftware ref your Style Magic files if you think they have become corrupt, they may be able to help.
In the circumstances, I would go for a Soft Reset as a base startline. Start from scratch. Unplug everything else and start with just the power lead and a single NEW usb in usb 001.
You MAY need to do a FULL FACTORY RESET if these steps haven't worked. See your manual for instructions
Its basic diagnostics, hopefully there may be something here to help you. Let us know how you get on!
*** If you still can't read anything even after a Factory Reset, then it's time to call your dealer.
Keith