Your INSERT syntax is wrong:
cmd = new MySqlCommand("INSERT INTO candidate(candidateImage,path where studentID ='" + Session["UserName"] + "') VALUES (@Name,@Path)", con1);
INSERT never has a WHERE clause - it can't because it doesn't return any rows, or affect any existing rows. Even if it did - which it doesn't - the WHERE clause is always at the end, not mashed into the middle of the values!
You would have to pass the StudentID as a column to be INSERTed along with the other data.
In addition: Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
DROP TABLE MyTable;
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?