Videos of certain lengths wont play

on my motorola android phone, I have a 32mb SD card, and if I record short videos its fine but when I record longer videos and I try to play them it displays an error message saying 'sorry this video cannot be played' they dont even have thumbnails, they look like grey videos even though at the beginning a little snip-it of video would work then display the message, I dont know what to do

 

I've tried downloading it but it still says sorry this video cannot be played, now its refusing to play any video

 

My SD card is fairly new but the weird thing is that it worked before but now its just suddenly stopped