You probably pissed a disc off. Potentially herniated it. Welcome to the club. I would not have pulled 400 on it, if I were you. For the first few days, take it easy. Lots of ibuprofen. After three days, start walking. See if you can squat the empty bar. Read Mac Ward's article on this site about disc herniation.
Back Rehab: A Case Study | Mac Ward
Back Injuries with Mac Ward | Starting Strength Podcast - YouTube
Back injuries suck and your next weeks and/or months may not be much fun. You will heal through movement and lifting, but don't be stupid. Provide it as much work as you can without making it worse. Heal this thing up and you will come back stronger.