A government shutdown occurs when Congress does not approve of or the President rejects the federal budget for the upcoming fiscal year. Since the 1990s, Congress has often failed to pass the twelve to thirteen appropriation bills that set government-wide spending.
In their stead they pass "continuing resolutions (CR)" to extend the existing spending law at or near current levels, and "omnibus" bills that combine many appropriations bills into one.
The National Emergencies Act (NEA) of 1976 allows the president to declare a national emergency, but he must outline the specific emergency powers he is using under existing statutes. According to the Brennan Center's running count, 32 of these are still in effect—including the ban on Iranian property, which was extended in November of 2018.