What should I do if my baby has day night reversal?

Get strict. Get very strict about social cues. Daytime: laughter, smiles, interaction, singing, lights on, activity. Night: keep it quiet and dim, with minimal interaction-- don't smile, don't play, don't make this time interesting at all. Keep day and night as different as can be to teach baby that there is a difference!
Limit day naps to 2h. Babies are capable of one deep sleep period/24h that lasts up to 4h. You can push this into the night if you are vigalent and limit baby to sleep/rest intervals of no more than 2h at a time during the day. After 3-4 days baby should get back on schedule. Babies sleep area should have soothing light and sound that promotes self settling & is used for day & night rest periods consistantly.