Hello,
Don't worry, there could be any number of reasons why he/she has not replied yet. When I was looking around PhDs I contacted lots of different universities and did not hear back from everyone I got in touch with straight away, or if at all. It's a good idea to send emails from a generic email address, and also to make sure your message is assertive, and explains why you have contacted this person specifically. If you know the institution you want to go to could you contact a postgraduate advisor/someone in admissions/or just any one else who might know who you could contact? Alternatively, you could send a polite follow up after a while (they might just be really busy). Good luck. M,x