Bootstrap + IE11 doesn't work. What is the reason?

Question

Web development

Good afternoon!
I downloaded the bootstrap files and included the styles in the header as usual. There was a problem with the fact that the bootstrap does not work in IE11, edge and on iphone 4s
When I connect from CDN, everything is fine
Two months ago, another site did it and there are no such problems.
In which direction to dig?

Answer 1

In the direction of the cache
In the direction of "look in the console, all of a sudden something fell off"
In the direction of checking the paths
In the direction of checking the bootstrap version
Well, basic things. It's hard to say something here without a link.