Accidentally managed to disallowed indexing, how to fix?

So I dun-goofed and disallowed indexing on one of my client sites, I’ve switched it back now but google has already stopped indexing the site.

Is there anything that I can do to repair the damage I’ve done?
If I switch it back is there any way to force a fresh crawl again ASAP?
Why-oh-why is this not a giant red shiny flashing warning on the dashboard telling me the site isn’t getting indexed?!

Any assistance would be very much appreciated.