It's really a tough situation when you have lost your registration number of IBPS. But, yes you can get back your number in following ways:

-IBPS send you IBPS registration number by email. You can see that email in your mail provider like gmail,yahoo etc.

In case, if you have entered invalid or wrong email address on IBPS form then it will be problem to get back your IBPS registration number.

But dint worry, IBPS also sends this registration number to your mobile which you have entered in IBPS online application form

In case if you don't find any registration numbers in the both above cases then you can mail in the following link;

callletter_helpdesk@ibpsorg.co.in