Why did my font change and how can I get my old ones back?

Gave my computer over to Geeks on Site yesterday. They ran about 10,000 scans and did a bunch of other stuff. Well, they also changed the font that I’m used to seeing. It’s like it went from Times New Roman to Ariel AND, worst of all, it’s smaller. How do I get my old font back? I could get used to it, but it’s harder to read. Like, the letters are spaced more closely together.
It’s weird, though. Some posts show my old font, other areqas show the new. For example, as I’m typing this it’s showing up in the new font that I hate, but the preview on the right shows my old font…..?