recover messages
how do I recover a specific message from my imac
What kind of message are you referring to (an email, a chat that came in via the Messages application, other)? The more info you can provide (model of iMac, version of macOS, what app you are using, the steps you've taken, etc) the more likely you will get a response that helps you find what you are looking for.
Without knowing exactly what you are looking for, I suggest reading this article about the Spotlight search tool built into macOS. It can be very helpful and has lots of search capabilities to help narrow down the search results.