Who should NOT use henna in their hair?
You should NOT put COMPOUND HENNA on hair that has been straightened, permed, dyed, or bleached in the previous year. The results can range from peculiar to catastrophic. Goat vomit green and swamp bottom black are the most common color results. If you use BODY ART QUALITY HENNA on your straightened, permed, dyed, or bleached hair, you'll probably do just fine, but test first!
If you have used ANY commercial hair dye in the previous year, harvest your hair from your hairbrush and henna your spare hair to see if you get the infamous green or if enough of the chemical residue has rinsed out that you get nice results!
Compound henna over Synthetically Dyed Hair = DEAD HAIR!
I do mean dead. You can't fix it.
This is "shave your head and join a nunnery" dead hair!
Start over. Grow it again.