in the 6 years you have had it.. have you ever changed the potting media .. it does wear out.. and the result is watering problems ...

if you want to find out about the roots.. then look at the roots ... tip it out of the pot and look at them.. and then just flop it back in ...

i suspect total and complete lack of deep watering ...

especially if when you have it out.. you realize its even drier on the bottom ...

look for nice fat happy roots ...

it you give it a very good drink.. i might give it a month.. to see if it resprouts .. and if it doesnt.. be done with it ...

check this out.. watch a few.. may as well try .. nothing to lose: https://duckduckgo.com/?q=plumeria+propagation&t=ffcm&iax=videos&ia=videos

personally.. i would not do it in water ...

lastly ... i dont 'do' plumeria.. but i would not expect a houseplant to go dormant ... has it done that ever before??... and if it hasnt/doesnt.. then its probably dead ... i will defer to those with more experience with them ...

ken