Adobe Reader DC won't show thumbnails in my documents on my computer

I have windows 10 home on my computer and in my documents there are pdf files that won't show the thumbnails.This happened once before and I fixed it but I can't remember how I did it.It was working fine but all of a sudden it just stopped working and I can't seem to get it to work again.I've tried watching some videoes on youtube but none of them worked.I just want all the pdf files to be so that I know what they are before I open them again.I'm attaching some pictures of what I'm talking about.I'm disabled and in a wheelchair from Rheumatoid Arthritis and I just want it back the way it was before.Thanks for any help that someone can give me,I really appreciate it.

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