DON’T MESSAGE THEM AGAIN.
I cannot stress this enough. Maybe they’ve decided not to keep talking to you - which is perhaps what you fear - but if this is the case, you’re just making yourself look pathetic. You won’t change their mind.
Or maybe, they’re just busy. Something came up. They’re asleep. They’re not online. They’re catching up with other messages. If you send a second message, you might start to seem a little…anxious at first. Desperate or clingy if it continues (or depending on the content of your message, it might just take one). And you might make the person decide not to message you anymore - even if before they had intended to, but just didn’t get around to it.
Now, you might suggest the possibility that they have forgotten to respond to your last message. Why send them a reminder? Fuck ‘em. They forgot to get back to you, which means, they weren’t all that interested in talking to you in the first place. Maybe, if you let it lie, they’ll eventually come back to it and realize they never responded to you (and remember how cool and irresistible and charming you were up until the moment they forgot they were talking to you) and send you a message - even months later - apologizing profusely and asking to start up the conversation again. That has happened to me. But it won’t happen if they come back to your message to find that you messaged them three, four, ten times with increasing desperation.
Just let it go. Don’t keep refreshing and wondering and overthinking the last thing you said. Message someone else. Or turn off OkCupid for a while.

File this under: advice I give out but don’t remember to follow.