Hello @Scott282,
It's not possible for a host to re-activate an expired request. All you can do is to try to contact the person and suggest that they send another request.
This hasn't happened to me so the following are just suggestions.
You could try going to your inbox to see if their request message is still there. If you can find it then just send them a message.
If the message is not there then you could try finding the reservation request email and seeing if there's any link in it that allows you to send a message.
Steve.