Dear Diksha, if you have generated the code earlier and you don't have the code, there are 2 ways to go about it. 

• You would have received the code to your registered Email and Mobile no# at the time of generstion. Yoy can use that and proceed with the final registration process. 
• But if you have lost those details, you can regenerate the code and then proceed with the final registration process. AIIMS has reopened the code generation wundiw from February 23rd and  you can generate the code until March 12th, 5.00pm IST.