The Rishonim debate [the nature of] the Omer count. According to the Behag, it is one long mitzvah stretching from Pesach to Shavu’ot, as it says, Seven weeks; they shall be complete (VaYikra 23:15). Therefore, one who forgets to count one day forfeits the mitzvah and may not count thereafter. However, most poskim maintain that each night’s count is a separate mitzvah. Hence, one who forgets to count one day loses that day’s mitzvah alone, and he must continue counting the next day, with a blessing (Tosafot, Rosh, Ritva, and more).
In practice, the halachic authorities have determined that even if one forgets to count an entire day, he must continue counting thereafter, in accordance with the majority opinion (that each day stands alone). However, he counts without a blessing, in deference to the opinion that holds that the entire count is one mitzvah (and when he misses a day, he forfeits the mitzvah). Thus, in order not to, possibly, recite a blessing in vain, he counts each subsequent day without a blessing (Sh.A. 489:8).
And in order not to forfeit the blessing, one who misses a day must have in mind to fulfill his obligation by way of hearing the cantor’s blessing.
This halachah demonstrates the tension that accompanies the Omer count. After all, one who skips a day breaks, to a certain degree, the chain that connects the holiday of Pesach to that of Shavu’ot, and forfeits the blessing. It is very important to connect Pesach, which represents holy Jewish nationalism, with Shavu’ot, the day we received the Torah, for Torah cannot exist without the Jewish people and the Jewish people cannot exist without Torah.
One could ask: why do we say a blessing every day according to the Behag? We are forced to say that even he admits that each day constitutes a separate mitzvah. It’s just that skipping a day mars the completeness of the count, making it impossible to continue counting. The Chida writes (Avodat HaKodesh 7:217) that one who knows [in advance] that he will miss a day – due to some uncontrollable circumstance – should omit the blessing from the start, because, according to the Behag, all of his blessings will be in vain. Most poskim, however, rule that he should count with a blessing until that day, because even the Behag (with whom the vast majority of Rishonim disagree) holds that the blessings made prior to the omission are not in vain (Kin’at Sofrim, Rav Poalim O.C. 3:32). This cannot be compared to a zavah’s count, about which Tosafot explain that no blessing is recited because if her count is nullified [by seeing new blood] the entire count is invalidated. [The difference being] that even if one forgets to count a day, the count continues and Shavu’ot arrives on the fiftieth day. Therefore, all the days that he counted without interruption were counted properly and the blessings [were warranted]. Only after missing a day, when the person himself will no longer count successively, does the Behag hold that [the blessing should be omitted because] the count is not “complete.” See Piskei Teshuvot 489:22.
One who forgot to count a day and needs to lead the prayers (because he has a yahrtzeit, for example) should ask someone else to recite the blessing and count [out loud], even though this will cause him discomfort [seeing that everyone will realize that he missed a day]. Others claim that in order to avoid embarrassment, one may rely on the majority of poskim who maintain that even one who forgets a day is obligated to count and may [even] recite the blessing on behalf of others. See Piskei Teshuvot 489:20.