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When I was feeling down, she cheered me up.
Literal
She [topic-は] I [subject-が] is-depressed [when-と] cheered-up for-me.
ふさぎ込む ('to be deeply depressed, to mope, to sink into the dumps') is a compound verb: ふさぐ ('to block, to plug up') + 込む ('into, fully'). The 込む intensifier is what makes it 'sunk deep into.' 元気づける ('to cheer up') is the causative-flavored 'make energetic.' The と here marks a temporal/conditional clause: 'when I was sinking, she...' — distinct from the quotative と. The sentence captures the recurring kindness of someone lifting another's spirits.