instead of the above i would recommend a pre-mixed clear paste. you can get it in gallon sizes.
it's really going to depend on the type of paper and if the seams will lay down as when originally hung.
depending on type of paper, i would brush some paste on the back of the paper and the wall, and then let it sit from 1-10 minutes to tack up. if it's just the paper came loose from the wall, it should lay down no prob. if it's the paper came loose, and also curled over, alot more fussin and cussin is involved;

and the paper may not lay down and the seams be tight like original. if it's some kind of vinyl paper and they did curl, you could try a heat gun to soften the vinyl so it will lay down. very carefully!
good luck- hangit