I'm guessing you're using Steam. They are known for randomly "losing" their link to Frontier. That's part of why I switched all my Steam commanders to Frontier-only.
To fix this, do the following (should be the same for Epic):
1. Completely close the third-party launcher you're using for ED
2. Login to your Frontier account here:
https://user.frontierstore.net/user/info
3. Unlink the third party account you're using for ED from there
4. Launch ED from the third party launcher, and it will allow you to renew the link
Your commander is stored on Frontier's server, so you will not lose anything.