When your back seizes up you should be very careful and then try to walk or crawl if possible to a chair or lay down on the floor.
Then use a heating pad if you have one to help apply heat to the back which can help your back and get it unlocked and if that doesn't work or you don't have a heating pad try applying some ice to the back.
If you cannot get the back pain to stop and get your back to unlock you should probably call 911 and get emergency services to you especially if you're unable to move around properly or walk.
Sometimes just sitting down with a heating pad or ice pack helps when the back seizes up but sometimes you may need to get medical help.