I accidenally deleted a supplier in the Supplier Onboarding module. Is there a way that I can restore them?

So long as you did not empty the recycle bin, then yes you can restore the supplier. 

To do so navigate to your Participant Summary tab within Supplier Onbaording and in the top right click on the Recycle Bin button. 

 

You will then be presented with a list of suppliers in the recyle bin. Find the relevant supplier and then click on the Restore button. 

This will then put the supplier back in your participant summary view and restore he supplier's access to the event.