If it is flexible, you can theoretically straighten it. If it won't bend at all due to being very hard wood, then there is nothing you can do about it. That is more likely to happen if you have a tree to straighten than a shrub though.

Also if it gets more sun on one side than the other, you won't be able to fill in the barer side but you could prune the sunnier side to give it some symmetry and let more light into the back side. Check other threads on when to prune because different types dictate pruning times unless you prefer a neat look versus blooms next year.

Not enough information to go on.

What kind is it? What region do you live in?