Incorrect syntax near '='.

You are doing it wrong — your query is composed by concatenation with strings taken from UI. Not only repeated string concatenation is inefficient (because strings are immutable; do I have to explain why it makes repeated concatenation bad?), but there is way more important issue: it opens the doors to a well-known exploit called SQL injection.

This is how it works: http://xkcd.com/327[^].

What to do? Just read about this problem and the main remedy: parametrized statements: http://en.wikipedia.org/wiki/SQL_injection[^].

With ADO.NET, use this: http://msdn.microsoft.com/en-us/library/ff648339.aspx[^].

Please see my past answers for some more detail:
EROR IN UPATE in com.ExecuteNonQuery();[^],
hi name is not displaying in name?[^].

—SA

Yeah, what Griff said.
Additionally, you should have used the debugger to show you the resultant CommandText -- the problem should have been obvious.
If it isn't obvious (and they aren't always), then you could have copied it and pasted it into SSMS and tried to execute it there.

I don't tink your technique of passing in the table, column, and key names are a good idea.
May I suggest that, if you insist on doing that, you use , params string[] Column) and pass in all the columns you want at once, rather than concatenating and hitting the database for each column?

Because you are returning at most one value, you may want to use ExecuteScalar rather than ExecuteReader.

But, as Griff said, you should validate the names. Because you seem to be using SQL Server, this can be as easy as querying the database catalogs.

Have a look at this, it ensures that the names passed into the method actually exist:

SELECT 'SELECT [' + D.name + '] FROM [' + A.name + '].[' + B.name + '] WHERE [' + C.name + '] = @key'
FROM sys.schemas A
INNER JOIN sys.objects B
ON A.schema_id=B.schema_id
INNER JOIN sys.columns C
ON B.Object_id=C.object_id
INNER JOIN sys.columns D
ON B.Object_id=D.object_id
WHERE A.name=@schemaname
AND B.name=@tablename AND B.type='U'
AND C.name=@keyname
AND D.name=@columnname

Use ExecuteScalar to execute something like that (with parameters). If the result is not DBNull.Value then you know the names were good and you can use the result as the CommandText.

Please, read what Sergey wrote in his answer and read my comment.

Instead of this:

C#

newCmd.CommandText = "SELECT" + Column + "FROM" + Table_Name + "WHERE" + Primary_key + "=" + RegNo + "";

use parameterized queries or even better (if it's SQL database) stored procedures[^].

See MSDN documentation:
Walkthrough: Using Only Stored Procedures (C#)[^]
How to: Execute a Stored Procedure that Returns No Value[^]
How to: Execute a Stored Procedure that Returns Rows[^]

Finally, i need to warn you. I'm not a follower such of queries, becasue of SQL Injection[^]!

SQL Injection and how to avoid it[^]
Stop SQL Injection Attacks Before They Stop You[^]
How To: Protect From SQL Injection in ASP.NET[^]