How do I make greek uppercase latters upright/roman again?

For some reason, when I last compiled my TEX code, all uppercase greek letters are suddenly no longer upright. Unfortunately, I don’t remember what exactly I changed. I found that the package upgreek is used, but I think that only creates alternative commands for upright greek letters. Can you give me hints of where to find these settings? The document’s setup TEX script is over 1000 lines long and I don’t even know where to start looking…

Additional info: I am using MiKTeX and PdfLaTeX. I only use the default fonts.