dishwasher is stuck in wash cycle and doesn't stop

It was installed new by seller 15 years ago and has never been used. I just have the plumber connect its pipe. New tenant started to use it and said it was stuck in wash cycle and doesn't stop.

What is possible reason?  How to fix it?

Do I have to buy a new one? but it had never been used and it is sitting there for 15 years.

Can anyone install the dishwasher or I have to have a plumber do it?

Thank you so much!

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