It would seem crazy to someone that has never had a student loan, but yes, it is very possible and extremely common to not be aware of all your loans after graduation. Since you can’t go back in time to tell 18-year-old you to keep track of every detail of each loan you take out, you’ll have to put in the legwork now.
For starters, check the National Student Loan Data System to find any federal loans. To check what you owe to private lenders, contact them directly. Another option is to order a free copy of your credit report to see who your lenders are.