12/12 messed up

I realized my switch didn't work and it worked 16 hours, do I start 12 hours from there or keep old cycle? Thanks

Either. I base my flip time on 12 hours of light giving me the most access to my plants. I go 6pm-6am. I can work on them in the evening and check them in the morning.

If your plants were in veg then go back to the normal light schedule you were using until they are ready for 12/12.
If your plants were in flower then go back to the normal light schedule you were using.

You should only set your timer to 12/12 when the plants are the size you want for flowering, if there's a hiccup in light or something the best thing you can do is go back to normal.

When moving plants from 18/6 or 24/0 to 12/12 it doesn't matter when you do it, the small difference in each method is not going to make a measurable difference.

Keep it simple.