First, check if ColorNote is on SD card. As the app doesn’t work properly from SD card, move the app to the device and reboot the device. FYI, the app automatically moves to SD card due to insufficient memory.

If ColorNote is not on the SD card, there might be a data problem in the device. Please uninstall and re-install ColorNote. And restore the notes backed up to SD card. (Before removing the app, check if your notes have been backed up to SD card using File manager app.) Or if you had signed up for online sync service, you can retrieve the notes from the cloud.