I recently shut down a business in Massachusetts. A large client refused to pay a final invoice consistent with our contract terms. Can I file a lawsuit on my own in Massachusetts to have this contract enforced, or must I have an attorney file it for me?

You certainly can file yourself, but it may be better to have an attorney help you out with this.

A lawyer (in CT) told me that a corporation had to have a lawyer file the lawsuit, and that you couldn't do it individually. Perhaps it's different in MA. I would use an attorney, but as we are shut down and have to collect these funds, it would have to be on a contingent basis.

I would suggest at least speaking to a couple attorneys to see what they can work out with you.