It is really sad that you have this situation with your partner. You can think a lot of different reasons about why this is happening. I’m not sure if it is possible for you to change their feelings and their relationship condition easily. Something else you should have in mind is that they might be hiding from you the reason for this “cold” relationship.
A good way to start is to talk to your partner about this. Discussing this might not change the condition, but that’s a small start. You have a long way to go, to see some real results.