That's been my challenge the last few days. Same thing happened a few years ago, caught me unawares on Monday. Improving with drugs and rest now.
I'd bent over very slightly while outside, and suddenly was hit by a pain in my lower back and then a muscle spasm so sharp I almost passed out. I made it into the house, found some good drugs for muscle spasm and pain, and collapsed on the couch. A few hours later the drugs wore off, and I was stuck downstairs with my salvation upstairs on the bathroom counter. Luckily I had my cell, and my son had his car at work, and his boss is understanding. My kids have both been a big help, making meals, doing dishes, feeding the cat - I couldn't manage on my own. After a few days of drugs and napping, I can at least stand a litle easier, and walk very carefully and very slowly downstairs - slightly tipping to the left (as in my politics).
I suspect this is related to a slight disc problem I've had in my lower back for years. I used to do some exercises designed to tighten the muscles around the spine - haven't lately. This is the price.
Meanwhile I lie in bed all day, surfing the net, sipping the occasional beer through a bendy straw.