First, check your battery, make sure it hasn’t corroded or damaged the calculator somehow.
Next, I think your best bet is to try to re-install the OS. There’s a guide on that here: Texas Instruments TI-Nspire CX OS Replacement
If that doesn’t help, try reaching the diagnostics menu or the maintenance menu by following one of these combinations I found on TI-Planet:
3) TI-Nspire CX/CM:
- [esc]+[menu]+[-]
- launches the diagnostic software, if present (non-dangerous choice)
- [doc]+[enter]+[EE]
- Goes to the Maintenance Menu (allowing you to delete the OS, etc.) (no effect if nothing if cancelled - not dangerous