The Mitzva of Sefirat Ha'omer requires counting the proper number each night during the Omer period. If a person forgot to count the Omer one night, then he should count during the following day without a Beracha, and thereafter continues counting each night with a Beracha, as usual. Even if a person forgot to count the Omer on several successive nights, so long as he counted during the day in each instance he continues counting the Omer with a Beracha. If, however, a person forgot to count the Omer one night and did not count at all during the following day, then he may no longer recite the Beracha over the counting of the Omer. He should continue counting each night, but without reciting a Beracha.
For this reason, many congregations have the practice to count the Omer aloud – without the Beracha – each morning during Shacharit, after the Kaddish following the Chazan's repetition. This is intended to ensure that anyone who forgot to count the Omer the previous night will at least count that day, so that he may resume counting the subsequent night with a Beracha. It should also be noted that although praying with a Minyan is of great importance throughout the year, it is particularly critical during the Sefira period, as one is far more likely to forget to count the Omer when he prays privately than when he prays with a Minyan.
If a person cannot remember whether or not he counted the Omer one night, and he did not count during the following day, does he continue to count with a Beracha, or must he count without a Beracha, given the possibility that he missed a day?
Chacham Ovadia Yosef, in his work Chazon Ovadia (Laws of Yom Tov, p. 238), rules that a person in such a case continues to count with a Beracha, as this situation involves a "Sefeik-Sefeika," or "double doubt." First, there is the question of whether or not the person indeed neglected to count the Omer. But in addition, even if he had neglected to count, it is unclear whether or not Halacha follows the opinion that one who misses a day of counting can no longer count the subsequent nights. According to some authorities, each night of the Omer constitutes an independent Mitzva, and thus forgetting to count one night does not affect one's obligation on the subsequent nights. Hence, in a situation where one is unsure whether or not he counted, two points of uncertainty are involved, in which case we may be lenient and allow the individual to continue counting the Omer with a Beracha.
Chacham Ovadia adds yet another factor, namely, the position of the Rif (Rabbi Yitzchak Alfasi, Morocco, 1013-1103) and the Rambam (Rabbi Moshe Maimonides, Spain-North Africa, 1135-1204) that counting the Omer constitutes a Torah obligation even nowadays. With regard to Torah law, we rule stringently in situations of uncertainty. Thus, according to the view of these authorities, a person who is unsure whether or not he must count the Omer is obligated to do so. Although we generally do not follow this position of the Rif and the Rambam, their view represents yet another consideration for requiring a person in such a situation to continue counting the Omer with a Beracha.
Summary: A person who forgets to the count the Omer one night should count during the following day without a Beracha, and then resume counting that night with a Beracha. If one forgets to count one night and does not count during the following day, either, then he resumes counting the subsequent night without reciting a Beracha. If one cannot remember whether or not he counted one day, he continues counting with a Beracha.