Do not ignore it
Default judgment is a real outcome, and setting one aside is harder and more expensive than filing a defence on time.
Check the date and the court
The document will state the court, the case number and the response deadline. Photograph every page, including the envelope.
Do not contact the other side first
Statements made before you understand the claim tend to appear in evidence later. Speak to a lawyer before responding.
Gather the file
The contract, invoices, correspondence and anything showing payments made. Completeness matters more than organisation at this stage.
Assess before you fight
Some claims should be defended, others settled early. That assessment is cheaper than the alternative in both directions.