artifacts in image / series of colored bars

[bartonlew]

I have been working in a very large file (12.3 GB) for some time with no issues.  Today I opened it and it is a series of colored bars.  Does anyone know what is going on here?  Thanks.

[bartonlew]

I should have added:  the image while I was working in it consisted of 7 layers; last night, I merged the layers, and this is the version showing here.  So it would seem that merging the layers has caused this behavior.  Why is that?  Thank you.