A user who signed up but never opened the product is a different problem from one who logged in but didn't complete the key action. The welcome email and the activation prompt both failed. Send a single short email that asks one direct question: "What got in the way of getting started?" One question is easier to answer than a re-onboarding pitch, and the reply often reveals a particular blocker: a confusing setup step, a feature they couldn't find, or simply that they signed up before they were ready.
Keep the email short, send it two to three days after signup if there's been no activity, and make no assumptions about what stopped them. If there's no reply, one follow-up near the end of the trial with a light offer (an extended trial, a setup call, or a simplified first step) is worth sending. After that, move on. A user who never opened the product during the trial is unlikely to convert without a meaningful change in their situation or your onboarding.