This is a pretty common questions, and what has happened here is that the lower teeth have started to overlap, and as they do so, they start to push the back of the upper teeth, making them go out of place as well!
Options for what to do:
So the problem here is tooth position, and there is (now) not enough room to keep the teeth straight. If the problem is quite mild, then you could have braces (either inside, or outside of the teeth), and this could straighten the teeth out again in about 6-12 months.
If there is a lot of overlap (less common), then its still possible to have braces, however the treatment is more complex, and might take 18 months or more.
Why has this happened?
A lot of people blame wisdom teeth for this, but that’s not what causes straight teeth to become crooked. As we get older, there is a natural tenancy for teeth to move towards the mid line, and sometimes, this causes teeth to overlap. This normally happens in the lower teeth, but as you can see here, the upper teeth are effected because the lowers knock the uppers out of position.