I've accidentally deleted a case, can I get it back?

Once a case has been deleted, there is no way of getting it back. However, a case can't be deleted if there are other entities associated with it such as documents, tasks, forms, notes, invoices, etc.

The only remnant left by the deleted case is its tracked activity in the History tab.

The suggested course of action is to create a new case and re-fill out the case details. You can do it yourself if you have the information, or assign the task to your client.