My WP website which I have been using for several months and working well, but suddenly today I cannot login to my admin area. I got some error message like attached. Even some regular pages of the website are displaying such texts.
It looks your website has been 'hacked'. Use FTP (or some other way to access the files) and try to open index.php (and some other PHP files) in the root folder with a text editor like Notepad+.
If it contains large strings with strange characters like your screenshot, someone has been editing your files. If this is the case, you better do a fresh install of Wordpress and update all your themes and plugins to the latest version.
I hope the cause is something different.