Sorry you have to be in top 20 percentile of respective board 

Bihar state board cutoff is 354/500

So you should have got 71 percent.

To get admission in IIT or nit

Anyway don't worry try for private colleges and state college

https://www.google.com/amp/s/engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission/amp