Hi {{first_name}},
You signed up for [[your product]] but it looks like you haven't had a chance to log in yet. I wanted to ask directly: what got in the way?
Was it a confusing setup step, a feature you couldn't find, or just not the right time? Even a one-line reply helps.
If you'd like a hand getting started, I can walk you through the setup on a quick call.
[[Your name]], [[your company]]