documents not opening properly

My mother recently change from medium to large icons on her computer and now her documents won't open properly. The icon for some of the documents looked like the icon for notepad. So I tried suggesting she open with notepad, but all she got was gibberish. I REALLY REALLY screwed up, it's my fault her computer is not opening documents properly. No one has ever screwed up as badly as this in this entire history of computers! I don't want to try open with again because I'll just make the problem worse. Can anyone tell me how to solve this major problem?

The computer operating system is windows 10 and it is a minisforum computer

At the moment I don't have access to that computer to test it. The problem happened after a new monitor was used. I looked for Microsoft word or open office earlier, which is what we usually use but could not find either. The files may have been originally created on a different computer, and moved over using a USB drive, but as far as I know they worked before the new monitor was attached.

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The icons were changed by going to the view tab in the folder and clicking on large icons.

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The files do open properly if open open office first and then open the file from there.

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They also open properly if I use the open with option and choose always open with open office

Need yo go into settings and re-associate the files with the right program.