tl;dr: reinstall the Windows 8.1 driver to get the scanner working.
I have a Canon MX850 I bought 8 years ago. It’s one of those cheap multifunction inkjet printer/scanner/fax things, the ones that cost $20 but each ink refill costs a small part of your immortal soul. The printer has been pretty reliable, but the scanner keeps failing. I’m not certain but my guess is that Canon’s network scanner driver isn’t smart enough to follow the scanner through IP address changes from DHCP. But the printer keeps working, huh.
Anyway, the simple fix is to download and reinstall Canon’s driver. No need to uninstall first. Windows 10 is not supported by Canon, but the 8.1 (x64) driver seems to work fine. At least scanning does, via Windows Fax and Scan. Haven’t tried the fax side.