This is a matter of targeted voting - AKA "serial voting".
Our network expects users to vote for posts based on their merits, not based on who wrote the post. When someone goes on a massive voting spree for a specific user, which is what happened in this case, those votes are generally caught by the system and automatically reverted.
You were given 20 upvotes within a very short period of time - 15 minutes - by the same user. This is clear targeted voting and not OK, so it was reverted by the system.
You (or anyone) can see the timing of the votes in your reputation history on your profile:
We have no way of knowing why someone voted this way but we like to assume they're doing it as a "thank you so much for your work" effort. Even then, this isn't acceptable behavior.
If someone wants to thank you for your help, they need to use the bounty system, which has the added bonus of allowing you to earn additional reputation that day because bounties aren't counted as part of the reputation cap.