You would likely have received statements with this number attached, and every time you contacted your bank, you probably would have had to reference this number too.
If you cannot find your old bank loan account number, the best thing to do is to contact the institution which granted you the loan to inquire about it.
You will probably have to provide the bank with documents confirming your identity, which may include a driver’s license or a passport. Some banks also accept cheque books as a form of ID.
Once the bank has this information, they will bring up your records, and provide you with the account number you are looking for.
You should be able to find your bank's phone contact details on their website.