[neonnoodle]

Try putting the notebook in the refrigerator or freezer for a few minutes.

[jbay808]

Unfortunately, this also brings back anything you deliberately erased, which might still render some writing unreadable!

[aaronius]

This helped me when I was using them some years back during my university time. Managed to place some papers on a heater and the ink disappeared. Putting it in the freezer for a bit made it all come back.