Why did I lose my top contributor logo under my avatar.?

If you do not continue to get the needed number of B/A in your category it will disappear.

I usually lose mine when I go on holiday, it comes back if you get a few answers chosen in the same category again.

✖ If you do not remain active and actively answer questions and getting Best Answers in the categories that you are a Top Contributor in, you can lose the badge. As was said before, so long as you stay active in the category(ies) that you want the Top Contributor badge in, and you keep on getting Best Answers in that category, you'll keep the Top Contributor badge.

₪ sɹǝʍsuɐ ooɥɐʎ ₪

You have probably participated less in the category of the TC badge you earned, thereby lessening the chances of getting best answers in that category. If you participate more and raise your best answer percentage, you will get it back. TC badges are given around 12:00AM Pacific Time each Monday morning.