Ok, first thing is to backup your entire site if possible. At least backup the database (which you should be able to do from the tools provided by your hosting company.)
Next thing is to start deactivating plugins. Try one at a time… and see if the full admin/dashboard returns. As I guess you can’t see the Plugins admin section due to the error you mentioned, you will have to deactivate the plugins via FTP (in /wp-content/plugins/). Renaming each plugin folder will do the trick. Just add a hyphen to the folder name and then reload the WordPress.
*** NB: after a plugin is deactivated, parts of you website will obviously stop functioning as intended. When you reactivate a plugin, you may need to know how to reconfigure them (depending on the plugin) if you don’t know, then you might not want to try this method.
If none of this helps, then you might have to try reinstalling WordPress. Try going to this page: {WordPress web root}/wp-admin/update-core.php
If you can’t see the Re-install Now button, then you will also have to reinstall via FTP.