You can use both of these methods but there are differences:

SELECT ISNULL(col1, 0 ) FROM table1
SELECT COALESCE(col1, 0 ) FROM table1

Comparing COALESCE() and ISNULL():

1. The ISNULL function and the COALESCE expression have a similar purpose but can behave differently.

2. Because ISNULL is a function, it is evaluated only once. As described above, the input values for the COALESCE expression can be evaluated multiple times.

3. Data type determination of the resulting expression is different. ISNULL uses the data type of the first parameter, COALESCE follows the CASE expression rules and returns the data type of value with the highest precedence.

4. The NULLability of the result expression is different for ISNULL and COALESCE. The ISNULL return value is always considered NOT NULLable (assuming the return value is a non-nullable one) whereas COALESCE with non-null parameters is considered to be NULL. So the expressions ISNULL(NULL, 1) and COALESCE(NULL, 1) although equivalent have different nullability values. This makes a difference if you are using these expressions in computed columns, creating key constraints or making the return value of a scalar UDF deterministic so that it can be indexed as shown in the following example.

-- This statement fails because the PRIMARY KEY cannot accept NULL values -- and the nullability of the COALESCE expression for col2 -- evaluates to NULL.

CREATE TABLE #Demo 
( 
    col1 integer NULL, 
    col2 AS COALESCE(col1, 0) PRIMARY KEY, 
    col3 AS ISNULL(col1, 0) 
); 

-- This statement succeeds because the nullability of the -- ISNULL function evaluates AS NOT NULL.

CREATE TABLE #Demo 
( 
    col1 integer NULL, 
    col2 AS COALESCE(col1, 0), 
    col3 AS ISNULL(col1, 0) PRIMARY KEY 
);

5. Validations for ISNULL and COALESCE are also different. For example, a NULL value for ISNULL is converted to int whereas for COALESCE, you must provide a data type.

6. ISNULL takes only 2 parameters whereas COALESCE takes a variable number of parameters.

   if you need to know more here is the full document from msdn.

You have two options really

ISNULL(yourColumn,0) AS columnName

or to actually fix the issue and change all the NULL data to 0

UPDATE table
SET column = 0
WHERE
column IS NULL    -- remember "column = NULL" always equates to NULL!

When you do this dont forget the WHERE or everything will end up being 0!

The coalesce() is the best solution when there are multiple columns [and]/[or] values and you want the first one. However, looking at books on-line, the query optimize converts it to a case statement.

MSDN excerpt

The COALESCE expression is a syntactic shortcut for the CASE expression.

That is, the code COALESCE(expression1,...n) is rewritten by the query optimizer as the following CASE expression:

CASE
   WHEN (expression1 IS NOT NULL) THEN expression1
   WHEN (expression2 IS NOT NULL) THEN expression2
   ...
   ELSE expressionN
END

With that said, why not a simple ISNULL()? Less code = better solution?

Here is a complete code snippet.

-- drop the test table
drop table #temp1
go

-- create test table
create table #temp1
(
    issue varchar(100) NOT NULL,
    total_amount int NULL
); 
go

-- create test data
insert into #temp1 values
    ('No nulls here', 12),
    ('I am a null', NULL);
go

-- isnull works fine
select
    isnull(total_amount, 0) as total_amount  
from #temp1

Last but not least, how are you getting null values into a NOT NULL column?

I had to change the table definition so that I could setup the test case. When I try to alter the table to NOT NULL, it fails since it does a nullability check.

-- this alter fails
alter table #temp1 alter column total_amount int NOT NULL

I would like to replace the total value to 0 when it is null

This is an impossible situation because:

The COUNT function always returns an integer. The result cannot be NULL!

As for coalescing an expression to a certain default in case it is NULL there are functions that do this in all major databases (ex.: COALESCE, NVL, ISNULL, IFNULL). The typical use is

FUNCTION_NAME(ExpressionThatMayBeNULL, DefaultWhenNull)

For specifics you should consult you database manufacturers documentation (you can find it online).

I am adding this answer because no one mentioned IFNULL function

You can use IFNULL

SELECT IFNULL(column_name, 0) FROM table_name;

IFNULL will return column's value (if it has something other than NULL) otherwise second parameter passed (in this case 0).