Have you ever logged into wordpress and went to go to the appearance editor and found that the link has gone? If so you should take a look at the other plugins on your website because it is more than likely that you may have the ” All in one Security ” plugin installed.
One thing about the all in one security plugin is that if you select some of ways that they suggest you secure your website then it will turn off the ability to use the editor. So if you have this plugin and you want to put your editor function back on under the appearance tab you simply have to go to the WP Security plugin, go down to the file systems security tab then click on the tab at the top that says WP file access as per the screenshot below.
Then you will see another screen that looks like below :
You simply have to untick the box, click save and your editor will return.
Many people tweaking their own website will have to go through the editor and amend their files accordingly. Without the editor access the only other way to fix this would be via FTP or downloading another CSS plugin which is a lot of hard work when you can just simply follow the steps above.
If you have any other questions or queries then feel free to get in touch.