Post preview not working

Hi there!
Last week I changed the permalinks on my blog, no major changes as I am not very familiar with coding and stuff, so I just changed the permalinks from Settings—> Permalinks. After doing so, I immediately changed back the permalinks to how they were before.
But the problem is, ever since I did this, I can not preview any drafts of any posts. Even the posts I had published long back, I can not view their “post preview”. When I click on the preview button, it opens the page in a new window but the page is blank. No error message shows, it is just blank.

Hope someone can help.

Thanks in advance!