Only thing you can do is very heavily water it to try to dilute the weed killer and hope

Thank You, I did water heavily and now I'm praying.

Good luck I hope you manage to save it!

If the weedkiller is a systemic one you might have a problem. Whenever I have done this I take off the wet greenery straight away to stop it drying and getting into the plant. If the strength of the poison was as indicated on the packet, it was allowed to dry and was done within a few days you might be unlucky. Usually there is a time limit, see packet, between putting on and washing off (raining) so if you have passed this time watering may not help. Best of luck with it anyway, sorry to be gloomy.