Let's start with easiest thing first...
Take out your SD card and re-insert it and see if your phone picks it up. (If you don't already know this is how you locate the SD card: http://www.fixya.com/support/t8475945-g1_memory_card_slot
If that doesn't work, you can try putting your SD card into a card reader (You may already have one on your computer. However, if you don't ask try asking a friend if they have one OR you can purchase one from here: http://www.amazon.com/Sandisk-MobileMate-Plus-Memory-Reader/dp/B000Q3IUV2
). If the card works in your computer try formatting it and putting it back into your phone. Still doesn't work? Try a different SD card in the phone. If that ALSO doesn't work then you likely have a bad card reader in your phone :'(
You'll have to call up your cell phone provider and see what they can do from there.
Hope that helps!