I was actually in the same situation until now. I didn't hear back from two of the professors I emailed, so after two weeks, I decided to contact them through other ways. I left messages for them with the department assistant and was able to talk to them not long afterwards. They told me since they get a lot of email everyday (with most being spam), mine fell through the cracks.
Everything worked out fine for me, so don't give up and move on yet! If they didn't want to write a letter for you, they would have responded to your email with a no. Most likely, they just missed your email.