Once you start a sprint, you are unable to put that sprint back into the unstarted state. If I accidently click the "start" button this gives me no way back to the former state. I'd like to be able to stop the sprint and re-start it again at the right time.
If you accidentally, start a sprint you can now reverse that mistake.
From the menu on the Sprint planning page select "Edit sprint details"
Hover over the status row on the details page the "Reset sprint" button will appear
Click the button to reset the sprint into a planning state.
This will erase all burndown information from the sprint. Additionally you can only reset the current active sprint, closed sprints can not be reopened.
The ability to look at historical sprints is already a feature. You can access this via the menu on the sprint planning page. This particular feature will allow you to reset an accidentally started sprint, so that you start it at the correct time.
Should be the same as Aha Roadmaps where you can reopen a release - there are always going to be times when you need to reopen a Sprint to review previous tickets let alone if someone accidentally closes a Sprint