Back-end of the website throws error when you mis-configure your plugin or theme or whenever there is some conflict.
Since you can access your homepage, it implies your server is up.
Open Filezilla and go to plugins folder
Rename the plugin folder to plugins.deactivate or plugins.abc (you may use anything).
And check whether you can access it or not.
Once you do this, and if you are able to open the dashboard, rename back to "plugins" itself.