Steve Rogers lost his powers due to having the Super-Soldier Serum drained from him by the villain Iron Nail.
This action aged Cap significantly. Steve called the Avengers together to tell them that Falcon would replace him as Captain America. Steve would still lead the Avengers as mission operation leader, but Sam Wilson was to replace him as Cap.
A cosmic cube was used to give Cap his powers back and de-age him, but this also started the Secret Empire event by rewriting reality in such a way that Captain America was an undercover Hydra operative the whole time.
