I did a cursory internet search and not much turned up. A full replacement main board may not be easy to come by. That leaves you with a handful of options:
# Take a roll of the dice and see if you can find a used one on eBay (I wouldn’t recommend this unless you have good relations with the seller)
# Contact Shark/Ninja and ask if they have the parts and get a quote/see if they will sell or give you one
# See if your ninja is still under warranty and file a claim
# Since the main board is relatively simple, determine which component has gone bad and replace it (i.e. resistors, capacitors, etc.)
I don’t recommend performing option number 4 unless you are very comfortable with electronics.
Best of luck! And remember to be safe when working with electricity and electric components.
[guide|25632|Here] is a guide for using a multi meter to test components on the PCB
[https://www.thomasnet.com/articles/automation-electronics/testing-pcb-components|Here] is a guide for testing different PCB components.