What do I do if the guy I like put his number in my phone randomly but didn't text me back?

It looks like he has a reason for not texting you any more. Mostly that sort of reason is female. Its very likely that he was texting a bunch of girls, but one in particular was more promising so he is now concentrating on her. And he just drops all the others. You included.

The world of electronic messages is very tough that way - all the social politeness that actual face to face talking requires is simply ignored.

I'm sorry, but he is just saying that you aren't the one for him. And he hasn't got the politeness or courage to say it to your face.

Keep looking. You'll find the one.