Most errors you'll see are related to the SD card --
The easiest thing to try is simply pull the SD card (actually, it's a slight "push" to get it to spring out - careful - it will fly out) and reinsert it.
If that doesn't work, try removing the SD card and using the EZ Scan software to format and prep the card.
After that, there is a "low level" format option....
Or, just pick up a new card (you'll want at least one backup SD card anyway).