Here's what I would do. Let's say the app is called "HPlan", so you have a \app\HPlan folder on your Essbase server. Within the HPlan folder there are several subfolders for each database, such as HPlan, Capex, Wrkforce, and so on.
- Stop the entire app using EAS
- Move the entire contents of the app\HPlan folder to a temporary folder
- Place the backup contents into the app\HPlan folder so that it looks the same as before. So if before you had \app\HPlan\HPlan\HPlan.otl and \app\HPlan\HPlan\HPlan.ind and so on, you would see those again.
- Start the app and see if it works.
- Try to login via Planning and see that it works.
This will only work assuming the app was never deleted from Hyperion Planning itself. If you recreated or created a new app in Planning itself then this won't work. In that case you would need to roll the database (SQL Server, Oracle, etc) back to a previous point so that you still had all of the metadata that's in Planning.
Good luck -- if this helps please mark it as helpful so that more people are encouraged to answer Essbase questions on Stack Overflow!