Font completely changed as I open the document

Hi! I updated the version of Libre Office (from 7. something to 25. something) and when opening a document, the whole font now is changed, which is dramatic.
I didn’t have problems 2 years ago modifying documents, the fonts were exact. What happened? And how can I get that font correct?

Thanks in advance for anybody wishing to guide me through it, I’m very much not an expert in informatics…

Have you updated your OS? This is where fonts are stored. Please, name your OS. How did you ascertain that fonts were substituted?

Some fonts might have been removed in the more recent versions. Click in a word and look at the font name in the font name box, if the name is shown in italics, then the font is not installed. You would then need to download the font, close LibreOffice completely, and install the font(s).

You can see a list of fonts on this page, Fonts - The Document Foundation Wiki