Check the trunk, particularly the base, and all the branches for signs of rot, infestation or disease. Its likely the problem is at the root - could be phytophthera root rot or verticillum wilt. If its either of those, don't plant another one there for a while, unless you replace the soil, both are fungal infections.

It looks as if the grass grows right up to the Acer so when mowing the trunk could have been damaged - follow Bamboos advice.

Thanks very much for your advise
I'll have a look , I'll be devestated if it
Has to go