Presuming from your name you are in UK. If this is a bamboo in the ground, then maybe it is just dropping its leaves prior to producing new ones. One of my bamboos had its leaves damaged in the late snow in April, but it is still very alive and will soon replace them. Just wait. Other main possibility is that the ground is waterlogged and it is drowning. Has your garden become a swamp in recent heavy rain? They don't like it swampy. Finally, there are some diseases/pests of bamboo, but they are quite rare in UK.

If it is in a pot, then perhaps it is telling you it is time to re-pot it, either to a larger pot, or split it in two and return to two pots the same size. Will need axe/wood saw to split it probably. You should really have done it a couple of months ago, but if it is suffering you'd better do it now.