The author of Hebrews is unknown, although many suspect that it might be Paul. We know that Hebrews had to have been written prior to A.D. 95, because Clement of Rome cites Hebrews in his Epistle to the Corinthians. WE also know that Timothy was still alive when Hebrews was written. And it is likely that it was written prior to A.D. 70, because the destruction of the Temple in A.D. 70 would have been a powerful argument supporting Christ's sacrifice replacing the oferings in the temple.

There is no evidence that eliminates an earlier date in the 40's. So a date anywhere within a range from A.D. 40 to 69 A.D. is possible.

Sources:
Encountering the New Testament, Walter A. Elwell / Robert W. Yarbrough, 1998, Baker Books, page 348

A Survey of the New Testament, Third Edition, Robert H. Gundry, 1994, A Survey of the New Testament, Third Edition, Robert H. Gundry, 1994, Zondervan, page 424