Confusion regarding Fajar Salah

My question is that I woke up for Fajar and turned off my alarm but something came I again slept...I don't know then again got up went to masjid there imam was in second rakat sitting...so he said salam . I prayed sunnah first then fard but one guy said your fard or sunnah are not accepted but when I left from home sun was already coming up so technically it was already when I arrived at the masjid...so would my salaah be okay because of the hadith that if you forget or sleep pray immediately.