If you are experiencing issues with the photo library accessed via the + button, please try the following:
Try Restarting Your Device
If your Photo Library uses iCloud, there may be an iCloud-related issue. Sometimes a full restart solves such problems. Click here to read how to restart your iPhone or iPad.
Still Not Working?
The + button is not the only way to add images. You can also use Copy/Paste, and additionally on iPad you can use drag & drop. See this page for instructions.