How do I get motivated to study hard after not getting good marks in 12th board?

I’d say don’t worry too much about being motivated. Lots of students are motivated to do well (especially at the beginning of a semester) but this doesn’t always mean that they will do well on exams. What you really need are studying behaviors that you can follow even when you aren’t motivated. This might be as simple as setting times in advance when you will read or review and then making sure that you stick to them, even when the motivation isn’t there or you don’t feel like you’re making progress. Learning how to do this will be far more rewarding than a temporarily burst of motivation will ever be. I hope this helps!