I could meditate once too, and then I couldn't. Turned out that my psycholocical issues (anxiety, OCD, etc) were stoping me from doing it. You may not know what is stopping you from meditating, but it may be something that is worrying you, an underlying issue, stress, an undiagnosed issue, or you are simply trying too hard.
I would recommend removing anything stressful (if you can) from your life and then try meditating again, from the bottom up as though you have never done it before, see if that does the trick.
It isn't that you cannot meditate, it is that something is stopping your mind from relaxing enough to get into a meditative state.
You can always try staring into a candle flame and going into a trance like state, the woman who taught me to meditate told me that everyone can meditate, they just don't realize they are doing it (the candle flame thing is one of the things she says everyone is able to do).
I hope that some of this rambelling has helped