I was trying to change a Master page on a site collection, but I keep getting the above error. Even when I revert to the default master page, the content of the site collection will not display.
You are on the master page preview screen. You should close the tab and type in the URL.
Its really confusing, I am not sure why there is no button to exit preview mode.
If you are still facing issue. Then restore a previous version as explained on this thread Stuck in "Preview Page" mode in SharePoint 2013
Fixed this by a workaround. I created another front page of the site collection and set it as a home page. Not too proud of it but since the version history in designer is not giving my access(it says :"server too busy"), I had to do it this way. This can be closed now.