What happened to Amtrak?

That's a very vague question.

Nothing in particular has "happened" to Amtrak; the company was created in 1971 in an attempt to revitalize the flagging passenger rail service in the US. It hasn't been spectacularly successful, but it also hasn't been so abysmal that the government has yet decided to completely give up.

Critics point out that Amtrak is not self-sufficient and requires significant subsidies from the federal government in order to continue operations. Proponents point out that so do competing forms of transportation, like highways and commercial airlines.

Amtrak has been forced to close some routes due to unprofitability. Train travel in the US is dying largely because there's no good reason for it. Airline travel is much faster (at roughly the same price for 21-day advance fares), and bus travel is no slower, usually noticeably less expensive, and generally offer more departure time options (many cities, even fairly major ones, in the US see only a single train per day in each direction, which may be inconveniently early in the morning or inconveniently late at night).

The main exception is the Northeast Corridor (Washington DC - Philadelphia PA - New York NY - New Haven CT - Boston MA), where the rail lines are maintained to a higher speed standard, and there are more frequent arrivals and departures. To a lesser extent, some major cities on or near the west coast have more frequent service (but, for the most part, only to other cities on or near the west coast), and Chicago is a major hub of the Amtrak system, with frequent arrivals and departures (though, again, usually only one or two per day to any other given destination).