I was serial downvoted yesterday, today I've earned my reputation back, but I discovered that I've got banned.
Was it because of the serial downvoting?
I think in your case, the reason for the question ban was the 6 questions you deleted over the last couple of days, seemingly in response to the serial downvoting. I've undeleted these, which should be enough to return you back over the threshold.
I've also issued a warning to the person targeting you with these downvotes, because you weren't the only one they were going after.
It's possible that the system that does the question banning and the system that does the question-unbanning are not 100% synced up with the voting system. There's likely some caching involved.
This should theoretically fix itself in a few hours or a few days, but if you can't wait that long, you could flag one of your posts for moderator attention, and then use the "Other" option to explain that you were serially downvoted, which triggered the ban, then when the votes were reversed, the ban stuck.
As others have mentioned, and as ChrisF mentions here, if you're that close to a question ban where some serial downvotes push you over the edge, you might consider using this opportunity to edit and beef up some of your posts. This will not only help reverse the ban but also help prevent another one in the future, should some unsavory individual decide to go on a downvoting rampage.