Halacha requires two bedikot per day lechatchila (the proper procedure), but if you forget one bedikah on day one or day seven (or even both bedikot on days two through six), you may continue counting.

You do not have to start over unless you neglected to do at least one bedikah on the first day and at least one on the seventh day. It is important to try and remember to do bedikot every day. That way, if you forget to check on the seventh day, you do not have to start again from day one, since days two and eight could be considered as days one and seven retroactively, and so forth.