The Epistle to the Laodiceans is a possible lost letter of Paul the Apostle, the original existence of which is inferred from the Epistle to the Colossians to send their letter to the church in Laodicea, and likewise obtain a copy of the letter "from Laodicea" (Colossians 4:16) Our knowledge of the letter to the Laodiceans is therefore dependent on our knowledge of the letter to the Colossians.
Richard DeMaris (The Colossian Controversy, page 11) cites Raymond Brown, who estimated that by 1984, around 60 per cent of scholars thought Colossians inauthentic. John Barclay says (Colossians and Philemon, page 22) Colossians is now routinely bracketed out as deutero-Pauline and that the consensus accepts only seven letters as 'assuredly' Pauline. This places our understanding of the Epistle to the Laodiceans in an entirely new light. Some have even argued that Laodicea was the intended destination of Colossians and that the reference in verse 4:16 was a device to ensure that Colossians would be received in Laodicea.
Wikipedia tells us that according to the Muratorian fragment, Marcion's canon contained an epistle called the Epistle to the Laodiceans which is commonly thought to be a forgery written to conform to his own point of view. However, this is not at all clear, since none of the text survives. If there ever was a letter to the Laodiceans, it is just possible that this was it.
For centuries some Western Latin Bibles used to contain a small Epistle from Paul to the Laodiceans. The oldest known Bible copy of this epistle is in a Fulda manuscript written for Victor of Capua in 546. It does not appear in any Greek copies of the Bible at all, nor is it known in Syriac or other versions. The text was explicitly rejected by Jerome and others in ancient times. This is unlikely to have been the epistle mentioned in the Epistle to the Colossians.