This situation is more common than you might think. You should start by talking with an expat tax expert to identify how many years of back taxes you are going to need to file and what documentation you need in order to complete the necessary reports and returns to become compliant with the US tax authorities.
You will need to prove expat status for each year. Be aware that you may owe penalties for filing late or failing to pay taxes on time.
Generally speaking, this will only be the case if you owed money on your US taxes, which will cause interest and penalties to accrue on the underpayment. If you have not declared your foreign bank accounts, you may also need to file FBAR forms for the years you have missed.