I would say 40 minutes isn't time to panic yet unless you don't know him very well or he has a history of doing this sort of ignore thing to you or previous partners.
I don't know where you live but it's late where I am. Is it possible he's fallen asleep after reading the message?
Even if that's not typical behavior for him it isn't out of the realm of possibilities.
Could something have genuinely come up and distracted him?
I know the U.S. has a lot of winter storms coming in so maybe it could be related to an emergency.
Is he busy and multitasking?
Sometimes when I know I can't reply I'll still read a message and think to myself that I'll come back and give it my full attention later. Maybe he's doing the same type of thing and doesn't realize how you feel his reply should be urgent and important.
If you seriously think he's just flat-out ignoring him then have you asked him that yet?
If not, I don't think it's terrible to send a text to ask him if he's purposely gone quiet.
Have you tried reaching out in other means like calling him?
I think if an hour passes and you've sent him at least one more text that you see he's read it isn't abnormal to give him a ring to see if he's just very busy.
When you get in contact with him again then maybe you should have a serious talk with him about your needs and expectations when you're feeling like you're in little space. I don't think it's a bad idea because maybe he doesn't think it's a big deal while you do.
Try to relax and distract yourself with something nice like warm cocoa and a stuffed animal cuddle.
Everything will work out okay in the end.