jasmin87,
If you are sure that you are not subject to US tax on that interest (i.e you haven't resided in the US for 5 years or more), you will need to file original or amended returns To the IRS to get those taxes refunded. You file an original return (1040NR) if you didn't file a return for a given tax year. You file an amended return (1040X) if you filed previously for a given tax year. As the deadline for requesting a tax refund is 3 years from the filing due date, you have until next April (3 years from April 2018) to complete those tasks.