I have just updated my Mac to 10.14.3 and found my scanner doesn't work. Be aware that, despite having accepted my money for the printer/scanner and it did work perfectly, they inform me they no longer support the latest Mac OS. Tough ! was their attitude.
I suggest you avoid this hardware.
iMac 27", 10.14
What OS did you have before you upgraded to Mojave. The last supported OS was High Sierra so I would download the drivers for High Sierra from this page, https://www.canon.co.uk/support/consumer_products/products/fax__multifunctionals/inkjet/pixma_mg_series/pixma_mg5250.aspx?type=drivers&language=EN&os=macOS%2010.13%20(High%20Sierra) there are two separate drivers to download, so scroll down the page, also click on Software at the top of the page.
Have you tried to scan using Image Capture which is in your Applications folder.
The next thing you could try is resetting the printing system, follow the instructions, here,
If you can't print from your Mac or iOS device - Apple Support
If all else fails you can try out Vuescan, https://www.hamrick.com
Upgrade Scanner - Canon MG5250
