Nothing will happen per se.
QlikView will keep working, until you run into a new bug or a platform incompatibility. After March 31st 2018, Qlik isn't obliged anymore to provide any form of support or updates for QlikView 11.20. They may, but they won't have to.
From a Business point-of-view, it isn't very good practice to pay for something (maintenance fees) when you're not getting any of it (except for the possibility to upgrade to 12.xx).
From a risk point-of-view, if after March 31st, 2018 QlikView 11.20 breaks down, you will be forced to perform an upgrade in a hurry before you'll get any help to fix the situation.
My 2cts: if your QlikView installation is business-critical, don't wait too long with your upgrade. Better plan for the 1st quarter of next year
