You didn't say what sort of phone you are using, but I am willing to guess that it is a Samsung and that the phone or CC has been recently upgraded. Several people have reported problems with Samsung not properly updating, with the result that SD cards don't work in CC or CC.
The fix is usually to uninstall and reinstall CC. See our FAQ
How do I reinstall CC without losing all my book information? for instructions. It is highly likely that you will re-tell CC which folder on the SD card contains your books (Settings / Formats and folders).
Sending me a CC debug log might help pinpoint what is going on with the SD card and missing reader problems. See
How do I get help? for instructions. Please erase the multipie.co.uk address if you send a log.