I'm talking from mobile here, but I think this is pretty similar on desktop:

If you click the hamburger menu at the far upper left (where it says "Win / TheDonald" at this site)...

... On that slide-out menu, there is a section for "Hidden."

When you're in there, if you click under any post where it says "Hidden" in red (where "Hide" normally would be), that will Un-hide any posts that were hidden.