Commercial Lease Issue - client won't pay for completed work

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Illustrative scenario prepared by my practice.
For informational purposes only. This is not legal advice. Laws vary by jurisdiction. Consult a qualified attorney for advice specific to your situation.
Hypothetical question

Sorry for the wall of text. Has anyone dealt with something like this? I'm not sure what my options are.

client won't pay for completed work. I've been dealing with this for about 2 weeks now and the situation isn't improving.

The contract was signed 16 months ago. I am not sure I have the original signed copy. The total amount in dispute is approximately $40,000.

Am I overthinking this or is this a real legal issue worth pursuing?

Illustrative response

This is really helpful, thank you. I've been dealing with a similar situation and didn't know where to start. Saved this thread for reference imo.