I doubt that any suit would be successful. No power provider I know gives more than a “best possible” commitment to provide power, not any firm guarantee. Pretty sure that the earthquake and tsunami would count as an accident of nature beyond Tepco’s control.
Sad thing is the loss of power along with the massive destruction of its farmland, fishing, and manufacturing facilities, along with its decimation of its people and their neighbourhoods because of the tsunami will take years to recover.
It is amazing the amount of attention that the "nuclear" part of this is getting given that the real devastation in terms of lives and the economy is the non-nuclear part.
And, not to make light of this, the overwhelming part of Japan's infrastructure and engine of productivity survived just fine.