Set WS = ThisWorkbook.Sheets(ComboBox1.Value)
WS.Select
Columns("AA").Select
    Set cell = Selection.Find(What:="celda", After:=ActiveCell, LookIn:=xlFormulas, _
        LookAt:=xlWhole, SearchOrder:=xlByRows, SearchDirection:=xlNext, _
        MatchCase:=False, SearchFormat:=False)
        ActiveCell.Offset(26, 1) = TextBox1



If cell Is Nothing Then
    'do it something

Else

End If