For heaven's sakes, how are we supposed to know? First of all, we'd need a lot more information about the sitch to even take a guess. Second, we're not him so we'd never be able to tell you for sure. Have you tried contacting him to ask him what's going on? That's the only surefire way to find out...
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Barring that, stuff happens: people get sick, people's parents or kids or siblings get sick and or die, people move suddenly, people's houses catch on fire. there are a lot of emergencies that might have kept this guy from getting in touch w/you. Also, people are often just plain old flaky. If you can't get in touch with him, you'll prolly never know...

Sometimes guys just disappear. Maybe his ex resurfaced. Or he met someone else...
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My brother told me once that if a guy doesn't call you for more than two weeks, he is definitely not interested...
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Break up with him in your head, and move on to someone who will show up...

U mean he's told you online or in person that he's looking forward to going on a date with you???..
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If it was online and u'd never met I'd guess one of two things :..he's either met someone else so not interested or not be on for some other reason travelling, internets down, illness, busy with his life..
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If it was in person he told u he's looking forward to going on a date, I'd likely guess the same two but put more weight on the 2nd reason for the disappearance act...
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But either way whatcha gonna do ... don't sit there pining for a mystery man ... Next...
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Good Luck..

He got distracted and clearly you were not a priority. What's more, he didn't bother to tell you. Sorry, but now you can look for a guy with better character than that...