Though the United States Federal Government has shutdown several times the current crisis marks the first during which the internet has become the primary means Federal agencies use to distribute information. The last time a government shutdown was looming there was a great deal of confusion as to how it might affect government websites. It looks like we are now finding out.
A government shutdown prevents the Federal Government from spending money on all but the most essential services, and it appears that for the time being maintaining many of its websites is not being deemed an essential service. The Burea of Economic Analysis, ERIC, and Census.gov (including all connected sites and services) are currently down. The FDA, CDC, and others are currently available, but many are displaying prominentt notices that sites are not currently being updated due to the shutdown and any information accessed may not be up to date.
USA.gov, which is currently up in a limited, non-updating capacity has a rundown of Federal agencies and services and the extent they will be operating for the duration of the shutdown.