First off, you should have multiple profitable campaigns going at all times for instances like this. Facebook will have inconsistent results with campaigns from time to time but as long as you have multiple campaigns that are crushing it, then you’re not dependent on one campaign.
To answer your question, if you only have one campaign running then I would let it run for a few days to see if it optimizes (3 days or so). If you can’t let it run for 3 days then just turn it off and restructure your campaigns to not be so fragile.
facebook is really unpredictable. I’d leave it 24hours and inform the client if things continue. it may be due to other reasons outside of facebooks algorithm. maybe things going on globally or seasonal times of the year ect. but i’d find that hard to be the case with a large spending budget.
I tend to find the campaign will make catch-up for poor performance at the end of the week as well. as many facebook users are on from wednesday – sunday.