Confused

So just when I was breathing a sigh of relief that all the canada hospital stuff was over and done and dusted, I just received a letter of Denial from my health insurance. I'm totally confused as they've already paid me a check for what I paid out of pocket, and paid off the Canada hospital for the treatments they billed for, so how can they deny me now? Can they now try and charge me what they've already given me a check for? And of course their offices are not open today to talk to someone about this either, which leaves me stressing about it all weekend......