Parts of Puerto Rico remained without power on Monday after Hurricane Fiona took out the entire electricity supply.
The storm left behind landslides and widespread flooding as it headed west to the Dominican Republic, with conditions too dangerous to make immediate repairs.
Heavy rain is expected to continue to batter Puerto Rico.
President Joe Biden declared a state of emergency for the US island, allowing authorities to provide disaster relief.
As well as a complete power outage for the 3.3m people living on the Caribbean island, some health centres running on generators were also affected.
Electrical systems in San Juan's hospital complex have since been restored, the health secretary said.
In some areas it will take days for the power to be reconnected, the operator of the island's grid, Luma Energy, said.
The Category 1 storm saw winds reach 86mph (140km/h). No deaths have been reported on Puerto Rico, but one person was killed on the French Caribbean island of Guadeloupe when his house was swept away by flooding.
Ports have been closed in Puerto Rico and flights out of the island's main airport have been cancelled.
Several roads are also closed and a highway bridge in Utuado, in the island's central mountainous region, has been washed away by flooding.
Puerto Rico's governor said schools and government agencies would remain closed on Monday. Pedro Pierluisi urged residents to seek shelter as soon as possible.
Hurricane Fiona hit Puerto Rico just five years after Hurricane Maria - the worst in the island's history - caused devastation.
Three weeks after that disaster, only about 10% of Puerto Ricans had electricity. The national grid still remains fragile, with power cuts a daily occurrence.
Torrential rains and mudslides are also forecast for the Dominican Republic as the hurricane progresses northwest, with the Turks and Caicos Islands also likely to be affected.
