How do you reach out to your users?

I'll answer your question two different ways.

First, how to talk to your users?

1. Do you have a welcome email upon signup? What does it say? Some services send an email, signed by the founder, that asks directly for feedback from the user. I've even seen some where the founder includes a link to an availability calendar, and invites the new user to book a 15-minute slot to talk about why they signed up, and how the founder can help the user accomplish that. Even though they've just signed up, they probably have some opinion of whether the service will be helpful, and they might reply, before they've given up entirely.
2. Do you have something like Intercom set up on your site? Some users might have questions on the site that they'd submit in a chat box if you had one.
3. If users are required to cancel (e.g. if they're paying for something, and need to cancel their account to stop you charging), then you can have a customer feedback form on the cancellation page. Though personally, I always skip those because I'm in the middle of trying to cancel my account, and don't want to take the time to give feedback. But I've also seen some services send a "sorry we lost you, can we do better?" email, and the emails might sometimes work.
4. If your service doesn't require cancellation, and users just drift away, it's going to be hard to reengage your users at that point. But what sort of email are you sending them? I think it's worth sending the email, even if you haven't gotten responses yet. Focus on the user and what they're trying to accomplish - your email could inquire whether they got what they wanted out of the service, and if not, what could be done better?

Second, maybe you can learn what you're trying to learn without talking to your users. If you're trying to learn why people are abandoning your service, you could also learn some of those answers by doing some user interviews. Even if someone has never used your service, if you confirm that they're in your target market, and then walk them through it and ask for their feedback, they might help you understand why your users aren't coming back.