JEE Main

There are many good engineering colleges in Indore, Bhopal and Jabalpur. You can look for any of the engineering college in Indore, Bhopal or Jabalpur.

It will be difficult for you to get into Nirma University with this score.

Yes, you can get it but you have to study B.Tech first year again.

Sorry dear, but 21 marks are not enough to grab you any govt. college.
You should try for any private college which offers admissions through direct means instead of JEE Mains rank and score.
All the best.
Please upvote my answer.

There are top private engineering colleges which conduct their own entrance exam like BITS-Pilani, VIT, SRM, UPES, etc., try these. Show your top performance and get selected.
All the best.

You won't be able to get any government college with this marks.

You won't get any decent college with this rank. Try other entrance exams if you have applied for any. If not, you can drop a year to prepare well but only if you are determined to work hard.

First go to this link: http://jeeadv.nic.in/jeeadvapp/root/PwdResetOptno.aspx
Then, select any one of the three ways to reset your password. Follow the further steps, you'll be able to reset the password.

If you are qualified, you can apply for NIT counselling through www.josaa.nic.in.
The same counselling is valid for all NITs, IIITs and also GFTIs.
To get the colleges other than NIT, IIIT, and GFTI, select the colleges where JEE Mains scores are accepted and just fill up your JEE roll no. and marks in the application form so that you can get admission.
All the best.

The cut-off score for ST category students is 27. Since you have not met the cut-off requirement, there are very less or no chances to get NITs, but if you can apply for counselling, try your luck. The counselling website is www.josaa.nic.in
All the best.