SharePoint (2016) subsite only visible in Internet Explorer and not in current browsers

Hello,

I'm not sure what happened or where to begin troubleshooting but it seems like one of my subsites has been reverted to an older UI maybe? Lost its CSS? Because when I try to access the site in a current browser (Edge, Chrome), it looks like there's no styling applied to the page but when I access it in Internet Explorer, the page works and everything looks normal except it looks like an older version or something of that sort...

I found an image online that someone else posted and seemed to have the same issue but there was nothing stated on how to really fix.

Please help?!

Thanks