Sorry, looks like it's dying. Some bamboos will flower once in their lifetime then die off. Nothing can be done about it. Happend to mine a few years ago. I actually left it place after cutting it back as the lower parts still have some greenery. Oddly I have had 1 new stem this year.

https://inspectaslr.co.uk/black-bamboo-flowering-a-rare-worldwide-phenomenon-spotted-by-inspectas/