This has happened to everyone, try this it worked for me hopefully for you too.
Go to My eBay Selling from drop down menu click on the Nectar logo.
Click on the button there to relink, then enter Nectar account, and Nectar password.
You will then be sent a passcode, enter this and it links the 2 accounts back up together again.
Use a desktop browser and not the app or mobile browser to do this. It is frustrating and it took me several attempts before it worked for me.
If it doesn't work I'm afraid you do ned to go back to Nectar Customer Services and see if they have a different workaround for you.