I took night time nyquil cold & flu before realized it expired 6 months ago. can this be harmful?

It is likely not harmful but it may not be as effective. Don’t take more of it if you notice a decreased effect. Throw the rest away! If you notice unusual symptoms, go to the ER or urgent care and get evaluated. The effects should wear off after about 8 hours or so-if you took it more than 8-12 hours ago, it is likely no longer having any effect. Buy a new bottle and only take as directed.