There can be many reasons for this.
Start off with checking the basics..
1) Is the contact in AllSubscribers list and in Active state.
2) Does the contact have a valid Email address.
3) Does the Data Extension have a field for Email that is EmailAddress type.
4) Is this Contact in any of the suppression or publication lists.
5) Is the Journey configuration using the correct Email address field.
6) Does Journey Builder history show you a message about this Contact, like Data Binding error or anything.
If all of these are checked and none of these are the reason, you can check with MC support to get help in finding out the reason.