I think it was the development costs of the C-series that has caused the problem, especially coming on top of reduced sales in the bizjet market.
That and spending over a billion developing the Learjet 85, which they've now had to write off and completely cancel, because the market for such midsize planes has shriveled, and a lot of the demand thats there is satisfied by used models or new copies of all the old designs still being produced at huge discounts, and the small niche of the market willing to pay real prices for new models is going to go for the Embraer 500 which is much more plane for about the same money.
Bombardier is screwed. They'll never get large C Series orders. The bottom half of their business jet model lineup has been completely surpassed by newer models from competitors. Their CRJ models are going to increasingly lose out to Embaer's E-jets. Their turboprop models will increasingly lose out to ATR.
Their only saving grace is the Global 7000/8000 project. Even with this, they have pushed back launch of the global 7000 2 years. And who knows how it will sell. The 7000 is the first purpose built business jet to be built with over 100' of length, which might scare off buyers if it prevents them from getting into their airports. And even if the program ends up going pretty well, would it be enough to save them from the disasters of their C-Series and Learjet programs? Maybe not.