Send to Peak Imaging or a similar develop and print laboratory (NOT high street chemists or retail photo outlet) and tell them to "pull" process by two stops (your errata is four times too much light).
They will adjust chemical strength and development time as best they can.
They will charge more for this individual attention, but not a fortune, a few dollars (£'s) per roll.
If you were using colour negative film then that has nearly a one stop leeway if conditions favour you (ie; you could have used 200 ISO and been nearly OK) so the extra stretch may be bearable, though some quality will be lost - they'll be slightly 'posterised' which means they will lack smooth tones from black to white and may also have colour shifts.
If they were trannies (slides for projection) they will suffer more and the highlights (the sky, any lights in the pix) will be 'blown' - they will likely be pure white. If the pictures are memorable, two stops is worth a try you'll get something out of them.
Consider scanning the results and spending hours in Photoshop retrieving some colour balance and so on. You may even with care, rescue highlight areas by substituting eg previously correctly taken skies, but the masking involved is generally laborious.
By cropping failed areas of the shots you might rescue many with very little work, it's amazing how little sky you actually need in a good pic.
Good luck, start with a tops process lab, they'll know what to do.