cement seems to be gone between blocks

This morning in the wind I noticed that my fence was shaking a lot, I can see that the the cement seems to have gone from btween the particular bricks underneath that section, What is the best remedy for this? I think the fence will snap if I don't do something fast. Would there be a ready for use cement or sealer I can buy that I can pour in between the cracks or what? thanks

I saw cement in buckets in Aldi last week - you mix it in the bucket. I know nothing about how you would fix your problem, i will leave that to the experts but if you need a small amount of cement this bucket would be a handy way of geting it, it was evostik brand.