Short Answer
Log on to Countewallet, in top left corner click on Create New Address > Create Regular Address to have it reappear.
Longer Answer
Because Counterwallet is a deterministic wallet, every wallet “knows” about all addresses it can possibly have, so the “missing” address (in reality, just one of few dozen addresses associated with the wallet that happens to not be displayed) should become visible again. Counterwallet addresses cannot “disappear” because they are cryptographically tied to the wallet which has a 1-to-1 relationship to the pass-phrase.
