Theme font has changed to Sergoe UI & Links are being underlined

Hello,
The font has changed on my site, even though the css still has the font-family as ‘Open Sans’ and it also open sans still in the functions.php

Upon inspection, I notice an error in the console that says:
ReferenceError: popper is not defined

There is also an red error next to this:
import $ from 'jquery'

This also is highlighted in the debugger:

(() => {
  if (typeof $ === 'undefined') {
    throw new Error('Bootstrap\'s JavaScript requires jQuery. jQuery must be included before Bootstrap\'s JavaScript.')
  }

Any ideas? I have no idea what this stuff is.
Thanks!