You could go for easier entrance exams such as VITEEE (For admission into the Vellore Institute of Technology) though the exam has already been done but still you can contact VIT people and take admission , Amrita Engineering Entrance Examination (AEEE; for admission into the Amrita School of Engineering) MU-OET (for admission into Manipal Institute of Technology), along with the above mentioned ones. Just don't register for too many exams as it could lead to frustration at that time.
Just dont sit back and cry for you could not clear mains. Many good engineering colleges are there you can opt for.
All the best

Dear aspirant,
Am sirry that you are not eligible to study b.tech this year.. .
You colud try and get better score next year.. .

Hey! Hi Prince Kumar

Just start preparing for upcoming exams so that you get through them into good colleges such as BITSAT, State CET's and others.

Hope it Helps.

Thanks.