Importing a file freezes the app at the ‘importing’ stage and restarting device or app doesn’t help.
The app is trying to accomplish / clear the pending operations in the import list and any issue with the file to be imported causes the app to hang at that stage.
In one of our previous versions (v6.1.12 to be precise), we provided an option to clear the import queue manually. This option is available under the iPad Settings app -> Noteshelf -> Reset Import Queue. Please update your app (if not done yet) and check if this setting helps open the app normally. If you have any questions, please reach our Support line via in-app email (Gear icon->Support->Email Support).