How to handle a lost key in Diffie-Hellman Key Exchange I implemented the Diffie-Hellman Key Exchange algorithm but, I was wondering how to handle if one of the parties lost his key.

Let's take a chat app as an example, Bob and Alice they used to have a shared key, but sadly Alice changed his device, and what if Bob sent a message to her encrypted with the shared key while she was offline. How can this be handled?

Not Sure about this solution:
Can I once Alice logs in with the new mobile device it requests from her room users to re-authenticate? and how about that message that Bob sent pre-re-authentication?

Thanks in advance. :)

EDIT ONE

As @kelalaka said, I can send an excuse to bob by re-auth and resending the message from his device.
You have to send the previously generated key securely and this is complicated and needs an additional protocol that you may not want. 

Otherwise, the key-exchange algorithm is randomized, and re-run will not produce the same key. Better is send a key-lost event, re-execute Diffie-Hellman and re-send the message.