Almost always, it is because you are not logging into a handle that is attached to a domain.
You may have several handles (without even knowing it), and are logging into one that is not a contact for the domain.
To make sure you're logging in to the correct handle, see Finding your Gandi handle(s)
If you've recently transferred a domain to Gandi, and have received notification that the transfer was successful but do not see the domain in your account, this is probably why:
Sometimes, it is necessary for our system to create a new handle to be listed as the owner of the domain in order to complete the transfer to Gandi.
When the information in the domain's Whois at the former registrar does not exactly match the information on the Gandi handle, this is considered a trade (change of registrar + change of owner) rather than a simple transfer (change of registrar).
If you do not select the “trade”/owner change option when launching the transfer, or if a trade is not allowed by the Registry in question, our system automatically generates a new Gandi handle which matches the Whois data.
Once the domain is at Gandi, you can reset the password to the automatically-generated handle and perform an ownership change to manage the domain from your primary Gandi handle.
Another reason why you may not see your domain, if you used the link above and see that you are using the right handle, or your domain does not appear, is that you did not renew your domain in time and it has gone into redemption period or has already been deleted.
You can verify that this by doing a Whois lookup or by checking the “Messages” tab in your Gandi account. If you have been sent multiple messages warning you that your domain is about to expire or be deleted, that's probably what happened.
To restore a domain that no longer appears in your account, use the renewal page. See also: Restoring a domain name
If you are unable to restore your domain, or do not fall into one of the above cases, please contact customer support.