I stubbed my toe and the toenail is coming off. what should i do and will it grow back?

Nothing fancy: How bad was the stubbing? If the results are leaving a bloody trauma site with the nail barely haanging on, then by all means cut the nail off as good as you can. If the nail is still pretty much secure, then tape the the nail down until the nail either loosens or grows enough to either trim the nail have it fall off by itself. You ca always seek professional help.