Basically, Russia, or shall we say, partially private company Gazprom, increased the price to the market level, removing the subsidies, that Ukraine benefited from over the years, and additionally demand payment for the previously unpaid deliveries, as well as enforcing penalties for missed payments.
It's business as usual. Market economy.

For a parallel think that you have a friendly grocer, who's been selling you bread for $1 instead of the market price of $1.5 over a few year's time, but you always really paid only $0.8 saying that you'd pay the rest later. As the grocer sees you as a friend, he says, "sure, no problem". Then one day you have a terrible row and call him a dick. The grocer is offended and demands that from this day on you pay $1.5 as everybody else, and on top of it return everything that you owe him from the previous purchases. You flip completely and threaten to go to court demanding that the grocer still supplies your bread for $1 because "it's always been like that".
Oh, and you can buy bread from other grocers, but you'd have to drive a few miles for that and end up paying $2 to them.

Yes, Ukraine has a choice of buying gas from Europe and US at market price, plus the mark-up that Europeans might take for re-selling that gas.

And to complete the analogy, one of the reasons for the row was that the grocer found out that you bought so much bread was not to "feed the pigeons", as you always stated, but to sell it on to your neighbours for $1.1, undercutting the grocer's business.