With the information provided there are several possibilities.
Last years decreased vigor might be due to a change in the soil ph. Pine needles are great for the acid loving plants but over time will likely lower the soil ph that would result in poor growing conditions for cannas.
Have you thinned to canna bed over the past couple of years? If not, the cannas likely became too thick for the available resources.
Did your rhizomes freeze or was the winter ground saturated with water? Any of a number of things could contribute to the total loss of the rhizomes.

Kent

could something have eaten them?

voles?

cannas were originally grown as a food source because of their high starch content. some animals might eat them if they came across the roots. moles might have chewed through them, but I think they are primarily carnivorous. my guess would be voles.

do you have a lot of sandy loose soil that attracts them?

if it is voles, you should be able to see their burrows if you dig deep enough... or at least their tunnels.