DB
In addition to what Fred has asked above:
Make sure you have not left a 3.5mm-to-1/4 inch headphone adapter in the headphone jack. That will cutoff the output to the speakers, even if a 3.5mm headphone plug is not connected to it.
Check the MIDI settings to ensure that LOCAL is NOT set to OFF.
If your problem only affects the built-in songs or the left hand accompaniment, see the right hand column of Page 47 in the manual for adjusting those volumes.
If you have no activity, at all - no lights, no sound, no display, then you may have a power supply problem. Make sure you are connecting it to a working service outlet, or to a power strip that is working and turned ON. If you are using batteries, they may need replacing.
The CTK-810IN does not appear to have a discrete "Factory Reset" feature, so if none of the above apply, and you have no sound from the speakers or headphones, then your CTK-810IN may need to go to a service center for further diagnostics.