I have already tried contacting the customer service, but ofcourse nobody is picking up
More than 500 euro would suggest you've had the 350 euro charge for theft of the bike plus the maximum of 21 days rent plus 5 euro per day extra as of day 4.
You should have received multiple reminders to bring the bike back.
As you state having returned the bike, probably something went wrong with returning. Did you lock the bike in the OV fiets zone (or with an old fashioned model return the key to the vault or the employee)?
We cannot solve the problem here, so you'll have to try again to contact customer service.
Good afternoon Sakshi and welcome to the NS Community.
How unfortunate that you received such a high invoice. Probably something went wrong when you brought the bicycle back to the station. I advise you to contact our Customer Service once again to check how this can be fixed.