LG Optimus Black stopped recognizing SD card

Hello friends,
I am using my phone for 6 months and everything had worked fine. I was making vidio and it saved to my SD card as usual. I stopped making and put my phone aside. A few minutes later I took my phone and shoot more videos and it says there is no external SD card.
I shut my phone down, removed the battery, and checked the SD and it was still inserted. I removed it and pushed it back in and it went in with no problems. I restarted my phone and it says there is no external SD card. What to do now?
I had never faced such problem at past. But as it has now occured I have to know its solution.
please help me out of this situation.