A Kansas Representative was recently pulled into the Trump team and handed his resignation in. We'll have a special election on April 11th for a replacement. Does my voter registration still count, or do I need to register again for this election?
If you were eligible to vote for the general election and you voted you will definitely not need to re-register for the special election. Some states may expire your registration if you do not vote for multiple election cycles.
I'm guessing you were referring to Senator Sessions from Alabama; I was not able to find any information on voter registration expiration there so I would guess it doesn't happen. You can contact them yourself to confirm though, their website is here