The picture is lovely, but exactly how and where is it turning brown? I. e., are the edges and tips of the leaves turning, or otherwise. Or, are individual branches involved, or the whole tree, or otherwise.
Also, have there been any changes in its life recently? Any digging or trenching near its base? Was there a change of grade? Were any of the neighboring plants removed, or pruned hard, removing its shelter?

Thank you but i may have misled you... i meantinstead of the bright red when i planted it years ago .it has gone dull in colour

Could you please let us have a photo of how it looks now

Yes, the picture we see looks like a healthy red Acer.