Upon completion of the White Walkers challenge, you do not receive your sprint back, nor can you do anything to get it back. Since completing the challenge, I have tried a variety of things such as; going into a new challenge, returning to spawn multiple times, leaving the server, going into theta, dying in maps and creative mode (and obviously /challenges quit). Still, no sprint is to be given. I am still currently unable to sprint. I don't know if it was just me somehow, but I would imagine it would be everyone who completes this challenge around this time.