。
Since she was cold, she turned on the heater.
Literal
She [topic-は] was-cold [because-ので] heater [object-を] turned-on.
ストーブ ('stove / heater') is a katakana loanword from English 'stove' but the meaning has drifted — in Japanese it almost always refers to a space heater (kerosene, electric, or gas) rather than a cooking stove. Cooking stoves are コンロ (also a loanword, from Dutch/Portuguese roots). ~ので gives the reason — softer and more explanatory than ~から. 寒かった (past form of 寒い) carries a subtle aspectual nuance: she felt cold, and as a result took action. Heating culture in Japan still leans heavily on space-by-space heating (kotatsu, room heaters) rather than central heating, which is rare in older homes — so 'turning on the heater' is a much more common winter action than in centrally-heated countries.