You mean what if the USA got involved in the Napoleonic Wars? 1812 was the US failed invasion of Canada resulting in what is described as a stalemate, despite Britain not only maintaining full naval dominance but also burning the white house and aiding Canada in its liberation.
If USA tried to help in the Napoleonic Wars nothing much would happen. The Royal Navy dominated the Atlantic so the US would have difficulty. The Royal Navy could blockade US ports or stay in the Atlantic for a long time thanks to naval doctrines. This was how Britain kept France in check.
A quote from Nelson: "Bonaparte has often made his boast that our fleet would be worn out by keeping the sea and that his was kept in order and increasing by staying in port; but know he finds, I fancy, if Emperors hear the truth, that his fleet suffers more in a night than ours in one year."
If the US did manage to somehow get to Europe how could they aid? It was mainly the winter that killed le grande armee so US troops would not help there. They could not invade already invaded land. And an attack on Britain would no doubt fail, just as it did when the French attempted in Ireland.
They could go to the peninsula however, considering the US relied on lots of militia, they would have a hard time doing anything against the British and Portugese veterans there; similar to how much they struggled against the heavy trained and disciplined peninsula veterans in the war of 1812.
In other words, the US would not be able to do anything major. They attempted to attack the Royal Navy and disrupt shipping lines to help France in reality but that failed too.