In IGNOU what if I fail to re register before the last day of registration?

Re-register before the registration deadline. If you are unable to do so, you will be unable to participate in the next semester for which you have registered. It will also give you a 6-month delay and allow you to register only on your next attempt, which is not a good idea. So be mindful of your re-registration and exam forms.

However, if you fail to re-register, don't be too concerned. Simply try to cover all previous exams and back from the previous semester and prepare for it. Learn some useful skills related to your course as well. Don't squander these six or twelve months; instead, get to work.