What do I do if I've accidentally deleted a donor?

It is not possible to 'delete' a donor that has any donation transactions associated with them. 

You can deactivate donors. This happens when you click the 'X' button to the right-hand side of the Manage Donors table.

At the top of the Manage Donor screen, if you change the radial button across to 'Deactivated', you will be able to see any 'deactivated' donors and reactivate them.

To better understand the Manage Donors module, please view the module overview here.