If you have hourly project "in progress" then you should be paid through hours tracked. But if you have a fixed price contract, then you can simply click on "Submit Milestone" button and you will be paid by 14th day!
I am assuming you have an active milestone if we are talking about fixed price project here.
re: "Clients don't respond."
Clients are NOT REQUIRED to respond.
re: "I finished work a few days ago. But the client doesn't respond."
The client does not need to respond. That is fine.
re: "I asked about pay."
I never ask about pay.
re: "He doesn't answer."
The client does not need to answer.
re: "What should I do?"
I do nothing.
After I click the "Submit Work" button, I just wait. And I get paid automatically.
If you have an hourly contract, you get paid automatically based on the time that you log. Clients don't need to respond. In any way. Ever.
On Fixed-Price jobs, freelancers are advised to use the submit work link on the contract details page in order to share the final product with their client and at the same moment request payment. If the client has simply stopped responding but the milestone was funded in escrow, then the client has a period of 14 days from the most recent work submission on a milestone to review and request any changes, after which the funds will automatically be released from escrow to you. You don't want to submit the work more than once, since this resets the clock back to the beginning and you will have to wait 14 days from your most recent submission for the funds to be released if the client does not take action on their end. You can have a look at this article about Fixed-Price Protection for more information.