What should I do if I am too late to check in ?
Once the check-in is closed, nothing can be done about this. Every day passengers miss their flight because they were late at check-in.
Usually the only solution to get to your destination is to buy a new ticket. Please bear in mind that tickets purchased for a flight on the same day are much more expensive in comparison than a ticket you booked a long time in advance.
Some ticket classes allow you to travel on an earlier or later flight. However, this is usually not the case for an economy ticket. If you wish to enjoy this kind of flexibility, you will have to purchase a ticket in another class which will usually be more expensive.