A well-functioning Data Model in Excel might start giving errors or stop working due to several reasons. Some of them are as under:

1. Large Datasets: If the Data Model is handling large datasets, it might exceed Excel’s capacity, causing it to fail.

2. Complex Relationships: If the Data Model has complex relationships between tables, it might lead to errors if these relationships are not managed properly.

3. Changed Column Names: DAX formulas require pre-specified column names. If the names are changed, or even a space is extra, it can lead to errors.

4. Data Issues: If the data required by a formula is not being referenced correctly, it can cause errors like the #N/A formula error.

5. Function Argument Errors: Errors can occur if the arguments of DAX functions are incorrect. For instance, the CALCULATE function requires filters defined as a Boolean expression or a table expression.

6. Data Type Conversion: Invalid data type conversion can lead to evaluation-time errors.

7. Changes in Data or Data Model: If the underlying data or the data model has changed, the DAX formula might stop working.

8. Software Updates: If Excel or the operating system has been updated recently, it could cause compatibility issues with the DAX formula.