In your case the following things could happen:
- May be memory card is not clean, so clean the card
- SD cards require stable power supply, make sure your battery is giving stable power
- May be metal wires in the card slot get rusty and twisted, so clean the metal part
- Card is not formatted properly, hence format the card properly
- May be card is broken, damaged
If these entire thing is correct and still your card is not read by your phone then you might have to send the phone to service center for repairing.
In the mean time check out – http://www.androiddata-recovery.com/blog/how-to-remove-issues-of-micro-sd-cards-from-android-devices
It will help you to restore all the data from the card.
May be this can help you.