The development of iMemos is discontinued some years ago. iMemos is not compatible with 64bit architecture required by iOS 11. As a replacement of iMemos, Apimac recommends to use iOS Notes.
To allow our users to retrieve their data, we created a tool (iMemos Reader) which is a special version of iMemos that let you read and share all the iMemos notes you have in your device, your should view it in the Upgrade tab of App Store, so just update iMemos, run it and you will be able to retrive your notes.