I sent an email asking for letter of recommendation almost a week ago and have not received a response. Does it mean NO? I asked my ex-professor for a LOR almost a week ago and I have got no response yet. It is making me pretty nervous and I am not sure what I should do now. I was thinking about visiting him at the department and asking him in person, but I am afraid that his silence means "no" and it will be quite an awkward situation when I ask him. 
I know that maybe he is on holiday, or the mail could fall into the spam folder, but I do not think so, because it never happened with any mail I sent him before. 

Is it good idea to see him in person and ask him like that? Can no response mean "no"? 

EDIT: He knows me - when he was my teacher we talked several times outside the class and I wrote him an email one month ago and he answered it, so I guess there shouldn't be a problem.
No, it doesn't mean "no". The most likely scenario is that your email is one of many waiting to be dealt with.

I would suggest politely and non-presumptively following up on the email. Reply to yourself to maintain the thread and include the date you need the letter by.