Accessing those files will be a bit hard, now you aren’t root on your phone anymore… 
The first requirement would be to have your computer detect your phone, which also isn’t the case at the moment. It might be the easiest, if you simply install Fairphone OS Kola Nut again on top of your current installation. The files in the /system partition will be replaced, giving you back the su-binary.
No user partitions will be harmed.
PS.: If you need an unrooted phone for work, you could try RootCloak, which is a module of the Xposed Framework.