There have a task which we assume be completed for a while, but just now we figured out that task is incomplete. How should I deal with that reopened task ? Should I set a baseline and then clean the task progress and set the new start/finish dates ?
I assume you have tasks that follow the task in question that had a false finish. I further assume those tasks have started and have progressed to some degree. You have to reschedule the work since you are essentially going back in time and continuing the offending task to get it to completion. So in your schedule, you would remove the finish date of this task and, based on your estimate to complete, you would add the hours, duration, and resources necessary to bring this task to finish. If those other tasks have to be restarted, i.e., the work performed so far has no value, then reduce progress on those tasks to 0%. Then march on with the work.
The question about rebaselining, however, is different. Whether or not you can rebaseline depends on why the task was claimed not to be finished. Did the workers on that task complete it based on the given requirements but the requirements changed? If so, that's a rebaseline, which means you get time and money added. If requirements were missed, then that would be a no rebaseline and you deal with your variances: late schedule, likely cost overrun. If the subsequent tasks revealed an issue with the offending task, then that might be a negotiable item as to whether it could be rebaselined, i.e., no reasonable person could have known that the task failed to deliver what it was supposed to.