Hmmm without ssh access it will be impossible to investigate this further. On nas central guides can be found to enable ssh on 5big2. You could try to accomplish that before consudering the following.
Sometimes a new install of the last OS (yes again) helps to solve this kind of problems. But I am not sure. Further there is in lacie firmware an option to reset the firmware, if I remember correctly, to the one installed by factory. But such an action can also end up in a disaster reading topics on the forum, resulting in a not working nas at all. So before attemping that I would suggest first to backup your data to another device. Just in case it goes wrong
About NFS, it is just another file server. What normally use is the samba server. This is used when accessing the nas for instance via windows explorer. You are probably not using NFS if you don't know what it is ;)