The correct usage is 'slept well'.
'Well' is an adverb; it is used to describe (give more information about) verbs. In your example, 'well' is describing the verb 'slept' (past tense of 'sleep'); you're talking about how you slept.
"How did you sleep?"
"I slept well."
'Good' is an adjective; it is used to describe nouns. You could re-write the sentence to use 'good', but it needs to describe a noun.
"Did you have a good sleep?"
"Yes, I had a good sleep, thank you."
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