If you work for one of America's major companies, your money is almost certainly safe.
Big plans have professional managers, conduct annual audits, and issue regular reports to employees.
You can usually pick your own investments. You might even be able to call the plan at any time, to find out what your account is worth. The same is true of most midsize companies, but there have been exceptions. Employees at Color Tile, which made floor coverings in Fort Worth, Texas, couldn't pick their own investments. The money was controlled by the company, and invested largely in company-owned real estate.