Another possible scenario if it is not a loose contact is that the FPC connector (the thin plastic ribbon with electrical contacts) that connects the removable storage daughterboard to the mainboard came loose. Carefully open the reader and inspect the contacts on the SD/MS slots and the tightness of the FPC ribbon connection. Here's a service guide for the PRS-600, which hopefully is similar enough to the 650:
http://www.ereaders.nl/prs-600.pdf