NASA, the Department of Justice and the Court of Appeals are among those whose sites have been affected, and the expired certificates have impacted services including payment portals, according to Netcraft.
Depending on your browser, you might be completely blocked from accessing affected sites, or see an error page warning that you're about to visit a potentially insecure site. Even if you can view the sites, they may not be processing requests or include the most up-to-date information.
With President Donald Trump and Democrats at a government funding impasse over the proposed border wall, it's unclear when furloughed federal IT employees will return and renew the TLS certificates. In the meantime, there's a chance that even more government sites could become effectively out of service if certificates weren't renewed in advance.