1. Use FTP to access your site
   bv. http://filezilla-project.org/download.php
2. rename all plugins by putting a ‘0’ in front of each plugin -folder;
   ../wp-content/plugins
3. Check if the access / ‘normal functioning’ has been restored
4. If so: one-by-one, remove the ‘0’, checking every time to see if the problem re-occurs.
5. when the problem re-occurs, you’ve found the source of the problem; Delete and find a similar plugin that works with your site
6. alternatively: FTP:Delete; Then download and install / activate a NEW version of the problematic plugin and see if this works.

Alternatively:
Instead of ftp, use the webaccess dashboard functions provided by the webhost.
This usually is not as versatile and more cumbersome.

There is also a plan B:

1.

1. Access to the server by my FTP (I’ve got a Mac, so I use Cyberduck instead of filezilla),
2. deactivate the .htaccess file on the server by renaming it to _.htacess.
3. then I was able to enter the admin area again,
4. there under settings/permalinks I refreshed the permalinks,
5. the refreshing creates automatically a new .htaccess fileon the server.
6. The lines automatically set there I deleted in the old .htaccess file,
7. then copied the rest of the content of the old _.htaccess file to the new .htaccess file.
Et voilà!