Error Establishing Database Connection

Hi on Monday, my website suddenly appears to be “Error Establishing Database Connection”, I thought I deleted the wp-config.php by mistake, so I follow the instruction on the forum and recreate one, and then the website and dashboard turned to be OK.

But yesterday I found that some of features of the theme I could not use. And I was told that it could be still the problem of wp-config.php.

So this time I restore the files. I have restored all the files to a week ago, but the website still on “Error Establishing Database Connection”, and the wp-admin page said my wp-config.php has something wrong.
But the files were a week ago and they worked exactly fine at that time.

Could someone tell me why this happened?
Thanks