On Thursday 7th May 2020, we reported that due to some technical issues, the CAMS Catalogue was temporarily not available. The issue has been fixed and we are pleased to inform our users that full access to the service has been restored: https://atmosphere.copernicus.eu/catalogue#/
Access to our CAMS data products is also available from this article in our Knowledge Base:
What data and charts are available through CAMS (Copernicus Atmosphere Monitoring Service)?
Copernicus Support team