Contact Your University or College:

Direct Approach: You can reach out to the administrative office of your university or college. They would be in possession of your hall ticket number.
Alumni Association: If your institution has an active alumni association, they might be able to provide you with your hall ticket number.
Scour Old Academic Documents and Certificates:

Search your old academic documents or certificates that carry your hall ticket number.
Go through your personal records, old files, boxes, or even digital backups.
Reach Out to Your Classmates:

They should give you a clue or lead you in the right direction to find the information.
Get Help from the Board of Examinations:

If you recall the name of the board that conducted your exam, say CBSE, ICSE, State Board, etc. Approach their office. Since they have the records or archives, they might be able to provide you with your hall ticket number.
Different educational institutions and boards have varying processes for accessing old academic records.
The most obvious step would be to initiate an action of contacting your university or college.