Question: I forgot to count the omer on Thursday night, and only remembered after davening kabbalas Shabbos. As that was before shekia, can I still continue counting with a beracha, or was is too late?
Answer: Tosafos (Menachos 66a) writes that there is a machlokes as to whether one may count the omer during the day, or if it must be done at night. Thus, the Shulchan Aruch (OC 489:7) writes that if one didn’t count at night, they should count during the day without reciting a beracha. They may, however, continue counting with a beracha on future nights (See Mishna Berura 489:34; Shaar Hatziyun 489:45).
The Taz (OC 600:2) writes about a community who hadn’t managed to fulfil the mitzva of hearing the shofar on the second day of Rosh Hashana that fell on a Friday. They davened kabbalas Shabbos early and then someone brought them a shofar. The Taz writes that in this scenario, even though they had already been mekabel Shabbos, they could still blow the shofar (See Taz OC 668).
R’ Moshe Feinstein (Igros Moshe OC 4:99:3) writes that this demonstrates that although one has brought Shabbos in early by saying kabbalas Shabbos, nonetheless, it is still the same day (Friday) regarding other halachos. One can, therefore, in this scenario, still count the omer and continue doing so later with a beracha. R’ Ovadia Yosef (Yabia Omer 4:43:8) brings multiple sources who agree that in this scenario one should count that day’s omer.In conclusion, while one has been mekabel Shabbos, it isn’t too late to still count the previous night’s omer.