@Mia146 Go to the review you are looking at for this guest. Click on the host's name under the host's profile photo next to that review. The host's profile will come up, scroll down to see their listings, then click on one of them (if there are several) to have their listing come up. Then go to the booking box, scroll down to the bottom where it says Contact Host. At least this is how I can do it in Google Chrome on my laptop.
If nothing comes up when you click on the host's name under their photo next to the review of their guest, that would mean that host is no longer active on the site.