First, there is a statute of limitations on claiming refunds. The IRS only allows a taxpayer to claim a refund for the 3 previous tax years. After that you're out of luck. And you will still be required to file your return.
Second, if you owe money the faster you file and pay the less penalties and interest you'll be assessed. If you are not able to pay in full you may be able to set up a payment plan. You'll still be charged interest but not penalties. Pay it off as quickly as possible.
Also, be prepared to pay for the tax prep. It won't be cheap to prepare multiple years at one time but it will be worth it. Don't be afraid to ask to make payments to your tax professional. They may be willing but they likely won't release any work product to you until payment has been received in full.