If you lost both power AND coupons, then someone else probably got onto your account and messed with it.

The game doesnt mess up that badly where you suddenly lose coupons. (The only problem I ever had was that I would not get stamina if I was not logged into the game)

Not having assist links on any of your ninjas does lose quite a bit of power if you have mostly high levels. For instance i have mostly level 9's and 8s on all my positions and if i take all of them off I lose about 20k.

If nobody got onto your account, you could have changed ninja lineups and some of your ninjas were not fully cultivated (Survival Trial cultivations), awakened, or full level. (as well as not having assist links if yours are high). Group summon also gives quite a bit of power (around 3k if all are well fed)

If someone got onto your account, check the pruifies, 8 inner gates, magatamas, and cave key runes. I had a friend with a jealous ex boyfriend go onto her account and wiped out 60k power (was able to get 10k back just from re splitting magatamas and cave keys again)