Try checking and unchecking box 2b to see if you can continue.
If the first box in 2b "Taxable amount not determined" is checked, the payer was unable to determine the taxable amount and box 2a should be blank, except for an IRA. TurboTax will help to determine the taxable amount.
If the second box in 2b "Total distribution" is checked, the distribution was a total distribution that closed out your account.