There is a less than 5% chance that they have lost their phone/phone was stolen, have been busy for a full 24 hours (so they couldn't even use their phone for 5 seconds, cause that's all it takes) or sent the message but the "delivery failed".
If it has been 24 hours and they have not replied, that's when you begin the 7 day reply test to confirm whether they want to text you or not.
Steve: I'm waiting on Leann to reply
Dan: How long has it been since u sent her a text?
Steve: More than 24 hours.....
Dan: Don't you know the 24 hour reply rule? She more than likely readeleted your text man, do the 7 day reply test to see if she really wants to text you