Help Center > Reputation & Moderation states:
If you see very unusual votes being targeted at your account, don't
worry about it. You should wait at least 24 hours after noticing
before becoming concerned, as the automated system should detect it
and reverse it for you. If, after 24 hours, you do not see any fix to
your reputation, please contact the team using the "contact us" form
located at the bottom of any page.
Related Stack Exchange Meta post:
What can I do if I'm the victim of serial downvoting?
Do NOTHING. The daily vote anomaly script will pick it up and your rep will be recalculated automatically.
If after 36 hours (give the script time to run) you still see a problem, then flag one of your posts and ask a moderator to look into
the anomalous voting patterns.
If it still hasn't been corrected, contact the SE team via the "contact us" link at the bottom of any page on the site.
Based on my experience, I flagged one of my own answers which was downvoted and asked a moderator to investigate it. I guess it led to suspension of one user who was serially downvoting my answers and others and surprisingly he had a few different accounts.