I just want to know what happens if I don't pay my Netflix account on time so it expires.
A new member month begins.
Then you no longer have access to the series. As simple as that.
You will then no longer be able to access films and series
Can I then top up the account with a voucher? So after the account has expired?
Yes, you can. The account is not gone, but only deactivated for the period until you top it up with credit.