I would speak to your vet. Whar stage is she in her season? Some bitches can get pregnant without the tie but it is unusual so you could be lucky. But there is an injection from the vet. Get her spayed asap if she is not!
Get her to the vet asap, she can have the jab which needs to be done fairly quickly. If its to late for that then they'll spay her. Slip matings are fairly common (where theres no tie) so its not worth the risk.