He volunteered to be a test subject for a super soldier serum, which was meant to create superhumans. He was then sent on a mission to stop the Red Skull and his attempts to weaponize a powerful energy source. During the mission, he crashed his plane into the Arctic Ocean to prevent the Red Skull from obtaining the energy source. The plane was frozen in the ice, and Captain America was in suspended animation.
He remained frozen for 70 years until he was found and revived by S.H.I.E.L.D. in the present day.