If you are a veteran who received disability severance payments after January 17, 1991, the DOD withheld taxes on those payments, and you reported the payments on your tax return, you are due a refund. You are supposed to receive detailed information from the DOD as to how to file for the refund.
If you have not received your information packet and believe that this situation pertains to you, you need to contact DFAS to obtain the necessary proof. You can file without the DOD letter but there will be some extra hoops to jump through. You can find more information from the IRS here: https://www.irs.gov/newsroom/veterans-owed-refunds-for-overpayments-attributable-to-disability-severance-payments-should-file-amended-returns-to-claim-tax-refunds
You only have one year from the date of your DOD letter to claim your refund. This time period is critical so do not put off filing. Follow all instructions and you should get your refund.