How can I add back Identity to a column?
Dropping the primary key constraint from a table does not alter the IDENTITY property of the primary key column.

USE tempdb;
DROP TABLE dbo.Question;
GO
CREATE TABLE dbo.Question
(
    QuestionID INT NOT NULL CONSTRAINT PK_Question PRIMARY KEY CLUSTERED IDENTITY(1,1)
    , SomeData VARCHAR(255) NULL
);

INSERT INTO dbo.Question (SomeData) VALUES ('test');

SELECT *
FROM dbo.Question;

ALTER TABLE dbo.Question DROP CONSTRAINT PK_Question;

INSERT INTO dbo.Question (SomeData) VALUES ('test2');

SELECT *
FROM dbo.Question;


The code above creates a table with a primary key, then drops the primary key.  It then adds a row to the table; and as you can see in the results below, the new row contains an auto-generated IDENTITY value for the QuestionID column.

Results:



Be aware that you could use the SQL Server Management GUI to modify the IDENTITY property of the column in question, however behind the scenes, SSMS will actually drop and recreate the table, which might be problematic to say the least.