How to Lock/Protect Cells in Google Sheets

Try this:

=iferror(vlookup(C2,Sheet2!A$1:B$6,2,FALSE),"")

Here's an example.

Some notes:

• Put the formula in the cell you want to fill
• C2 refers to the value you're using to look up in the array
• When entering the array range, make sure to put in $ before the row numbers, otherwise the range will be dynamic, not fixed.
• If you add more categories, you'll need to update the formula on Sheet1

I've created a script that does what you ask for. But since you can't invoke "Undo" with script it starts with copying your data to a separate sheet (called "scriptSheet"), which you have to create. You can hide it after that. This copying happens every time you open your spreadsheet.

When you edit a cell in a specified range the script checks if a date and time specified by you has passed, if it has it informs the user and copies the value from the scriptSheet. If the date hasn't passed the value is instead copied into the scriptSheet.

So here's the script:

function onOpen() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheetByName("Locked Sheet");        //Edit to be the name of your sheet
  var scriptSheet = ss.getSheetByName("scriptSheet");
  var lastRow = sheet.getLastRow();
  var lastCol = sheet.getLastColumn();
  scriptSheet.clear();
  sheet.getRange(1, 1, lastRow, lastCol).copyTo(scriptSheet.getRange(1, 1));
}

function onEdit(){
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getActiveSheet();
  var range = sheet.getActiveRange();
  var currRow = range.getRow();
  var currCol = range.getColumn();

  if(sheet.getName() == "Locked Sheet" && currCol == 1){ //Edit to be the name of your sheet and column with dropdowns
    var lockDateRange = sheet.getRange(2, 6);            //Edit to be the address of the cell with your date or set to string, '3/16/2013 17:00:00' for example
    var scriptSheet = ss.getSheetByName("scriptSheet");
    var now = new Date();
    var lockDate = new Date(lockDateRange.getValue());
    if (now > lockDate){
      scriptSheet.getRange(currRow, currCol).copyTo(sheet.getRange(currRow, currCol));
      Browser.msgBox("You do not have permission to edit this cell");
    }else{
      sheet.getRange(currRow, currCol).copyTo(scriptSheet.getRange(currRow, currCol));
    }
  }
}

Right now the date is retrieved from a cell (F2) as stated in the comment on row 19 you can write the date in the script instead. The cell reference just makes it easier to change the date.