Because the check was sent to you and was cashable (at least for a while), it is considered to be received by you (doctrine of constructive receipt) even if you did not cash it. Therefore, it is taxable to you since you did not roll it over to a new account within 60 days. For the same reason (constructive receipt) I believe John is correct that the 401k withdrawal can't just be "canceled" with the money returned to tax-free status (not after 60 days). It is probably in a holding or escrow account until you deposit the check or ask for it to be reissued.
[Edited to add] Although you can't cancel the withdrawal, you may be able to waive the 60 day rollover rule and complete the rollover by depositing the check into an IRA or other qualified plan, if the plan administrator will accept it. See Revenue procedure 2016-47.
