There have been problems with the Sentinel-3A SLSTR instrument in the period between 3rd and 12th of February 2020. Read more here: https://scihub.copernicus.eu/news/News00659
It has been reported that the cause of the problems is related to high temperatures within the SLSTR instrument. The IR channels were switched off and other channels (VIS) were downgraded during this period due to calibration errors caused by the high temperature. Everything seems to be working well now and the Sentinel-3 team is working to find the original cause of the temperature increase.