According to the report on the English pages of Der Spiegel, the 1971 building collapsed. Work on the tube line running under the front wall may be responsible. Loads of documents may be lost, some dating back to 922 AD. It’s unclear whether any medieval manuscripts will have gone west. More here (in German), which links to a PDF showing pages of a 9/10th century gospel manuscript on p.20 of the PDF.
I bet that they didn’t allow anyone to photograph them first. In most disasters, the fire brigade are called in to saturate the building with water, just in case any documents escaped. That usually finishes them off!
UPDATE: The Cologne Archive is currently asking for volunteers in the area to help with the disaster recovery. If you are interested in helping out, please see the archive website: http://www.koelner-stadtarchiv.de/index.html