@AnnLoomis, @ruiz_ashley, @luv2retrieve, and @JohnPatkin ...
This is interesting! I just re-visited my Grades page in my Canvas sandbox course, and I can also reproduce the issue that you all are seeing. The notes that I had entered yesterday are completely gone now from that view, and if I click on a cell for a student, there is no text where I had originally entered text yesterday. When I exported the Grades to a CSV file, the comments were there. I also tried switch back and forth between the current Grades view and the Enhanced Gradebook Filters view to see if that made any difference, but it did not.
I would suggest that you report this to the Canvas Help Desk staff as soon as possible so that you can log a ticket with them. If it is a bug that they need to fix, you'll be updated as soon as a solution is put in place.
I hope this will be of some help! Thanks to everyone who also reported this.