Thank you for your message. When you submit your work on a Fixed price contract using 'Submit work for Payment' button, your client will review and either approve the milestone or ask for changes. The client has 14 days from your submission on a milestone to review and approve or request changes. If they do not respond to any milestone submission within 14 days, the deposit is automatically released to the freelancer. Check out this help page for more information.
re: "in a fixed contract job if a freelancer submit a work done and the client did not respond or approve payment what happened next"
It doesn't matter if the client responds.
Clients are not required to respond.
As a freelancer, I get paid the same whether or not a client responds.
If a client doesn't respond, then I get paid automatically.