It is entirely possible that it is taxable.
If box 2a is blank (there's nothing in there), that doesn't mean the box 1 amount is nontaxable. Rather, it is up to you to determine from your records the nontaxable amount to enter in box 2a.
If this was from a pension and you made after tax contributions, then there would be an amount 5 or 9. The portion of your payment(s) that is not taxable is shown in Box 5 of the 1099-R. It is this amount that the IRS considers a return of your previously taxed contributions.
Box 9b on the 1099-R shows the amount of the employee "contribution" to the retirement plan. "Basis" in a retirement plan is also called "cost" or "contribution". In a word, it is the amount of after-tax dollars that the taxpayer contributed to the retirement plan over the years while he/she was employed.
If it is for an IRA, you would need to know any nondeductible contributions.
For a Roth:
When you are entering this information into TurboTax, your Form 1099-R, box 7 codes J, Q and T identifies a Roth IRA distribution and determines the tax treatment. If you have a J or a T, the distribution is considered taxable unless there is an exception. TurboTax will guide you on all the exceptions.
