If you entered a Form 1099-R with an amount in box 1 of the form and an amount in box 2a of the form those amounts are entered on your Form 1040.
If this was from a 401(k), annuity or other tax deferred retirement account, not an IRA, the the amount from box 1 is entered on the Form 1040 Line 5a and the taxable amount from box 2a is entered on Line 5b.
If this is from an IRA the taxable amount from box 2a is entered on the Form 1040 Line 4b. Line 4a will be blank on the Form 1040 unless there is an exception as noted in the Form 1040 Instruction for Line 4a on page 26 - https://www.irs.gov/pub/irs-pdf/i1040gi.pdf#pahe=26