It is well-known that if a person forgot to count one night of the Omer, he should count the following day, without a Beracha, and he then resumes counting that night with a Beracha. Thus, for example, if a person forgot to count on the twenty-second night of the Omer, then the following day he should count "Hayom Shenayim Ve’esrim Yom La’omer…" without reciting a Beracha. (Our custom is to make a public counting of the Omer each morning during Shaharit, before Ashreh.) He then counts the twenty-third day of the Omer that night as usual, with a Beracha.
If, however, one misses an entire day of counting, then he no longer counts the Omer with a Beracha. This means that if a person did not count at all at night or the following day, he must omit the Beracha when counting the Omer henceforth. There is a common misconception that once a person misses a day of counting, he no longer needs to count at all. This is not correct; a person in such a situation must continue counting each night, only without reciting a Beracha. There are Halachic authorities who maintain that the obligation of Sefirat Ha’omer remains fully intact even after one misses a day of counting, and one must count even with a Beracha. We omit the Beracha in this situation in deference to the view that there is no longer an obligation of counting once one missed a day, but one must nevertheless continue counting as required according to many authorities.
Hacham Ben Sion Abba Shaul (Israel, 1923-1998), in his work Or Le’sion (vol. 3, 16:7), writes that when one resumes counting after missing a day, he must first make mention of the missed day. For example, if one missed the twenty-second day of the Omer, he cannot simply count that night "Hayom Shelosha Ve’esrim Yom La’omer She’hem Shelosha Shabuot U’shneh Yamim." Since he had missed the twenty-second day, he cannot "jump" to the twenty-third day. Instead, he must first say, "Etmol Haya Shenayim Ve’esrim Yom La’omer She’hem Shelosha Shabuot Ve’yom Ehad," noting the previous day’s counting, and then he can proceed to count the twenty-third day. And if a person missed two days of counting, then he must mention both days he had missed ("Shilshom Haya… Etmol Haya…"). No matter how many successive days one missed, he must mention all of them before proceeding to count that night of the Omer.
Summary: A person who missed a day of counting during the Omer must continue counting the Omer each night thereafter, though without a Beracha. When one resumes counting after missing a day, he must first mention the previous day’s counting by saying "Etmol Haya…La’omer," and only then proceed to that night’s counting.