If it doesn't turn off then it's very probably waiting for something to happen that is not happening.
Given that you say the washing cycle is completed then there's not much that it can be - the last thing the machine does is to drain.
Either the draining process is not completing or the sensor or timer that decides to turn the machine off is defective.
You may do best to either...
Turn it off yourself - no real hardship
Have an engineer call - there will be a cost involved.