Suddenly, I am unable to open any of the papers. I tried refreshing the page and then I could only see a black screen. None of the papers or folders are visible now
This is a generic storage I/O error. There's no way for us to tell you specifically what the problem is, but the most common cause is having your Zotero data directory on a network filesystem or in a cloud storage folder (where it should never be). It could also be due to an actual physical problem with your disk or due to security software interfering with Zotero's ability to access the disk.
(You also have an outdated version of Zotero 7, so you should reinstall from the download page, but that wouldn't be causing this.)
Upgrade Storage
(You also have an outdated version of Zotero 7, so you should reinstall from the download page, but that wouldn't be causing this.)