The recent government shutdown has caused the link to the NGS CORS base stations to be temporarily disabled. This means that you will not be able to access any correction data coming from this source until the shutdown ends.
CORS stations will continue to log data, but it will not be available until the government shutdown comes to an end. There are other correction sources (privately owned base stations and VRS networks) that should still work.
If you are having trouble differentially correcting data, this may be the reason.
The CompassTools base station in Denver is still operating as normal.