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Let yourself feel it for a day. Being rejected after preparing for weeks is disappointing, and pretending otherwise does not help. A walk, a conversation with a friend, a game: whatever helps. Then do the next steps while the interview is still fresh.
Write down what happened. On a page, list each round, the questions you remember, the ones you answered well, the ones you did not, and anything that surprised you. Do it the same day or the next. Memory fades quickly, and this page is the most useful thing you will have for the next drive.
Ask for feedback, politely. Your placement cell may have some from the company. If the interviewer was friendly, a short, polite message thanking them and asking if they have any suggestions sometimes gets a reply. Many companies will not give feedback, and that is not personal. If you get none, your own notes are enough.
Look for patterns across more than one rejection, not a single one. One poor interview can be a bad day. If you were stopped at the aptitude round three times, the aptitude round is the thing to work on. If you were stopped at the technical round, the fundamentals are. Patterns tell you what to practise.
Choose one thing to fix before the next drive. Not ten. 'Practise explaining my project aloud twice a day' or 'do three timed aptitude tests this week.' A single clear goal is more likely to happen, and small improvements add up across the next few drives.
Talk to someone who has been through it. A senior who was placed last year will probably tell you they were rejected several times first. Hearing that from a person is more convincing than reading it.
Keep going. Waiting until you feel ready before applying again is a trap, because you improve most by doing real interviews. Apply to the next drive you are eligible for, with your one fix in place.
Look after yourself as well. Placement season can be stressful, particularly when classmates are being selected and you are not. If you find you cannot sleep, cannot concentrate or feel hopeless for more than a few days, talk to someone: a family member, a friend, a faculty member or your college's counsellor. That is not a weakness, and it is far more important than any single offer.