My nasal bone moved. How can I straighten it back?

the nasal bones are fixed, and unlike cartilage, don't tend to warp spontaneously.  So I don't really know why your nasal structure changed.  I suspect that the depression you see is more likely from a depressed or partially avulsed upper lateral cartilage as it articulates or meets the nasal bone.  sometimes prior trauma will finally manifest like this, even if you did not pinpoint an exact event.

In any event, you have a number of options, which is why you need to see a rhinoplasty physician.  Then he or she can evaluate the etiology and give you options.  They might include a simple injection of filler to camoflage that depression, more permanent soft tissue fillers such as your own fat or fascia, or even a procedure involving precisely shaped and softened cartilage.  All those might camoflage your irregularity.

The other solution is clearly a rhinoplasty, which would not just camoflage what you see, but actually correct the underlying source.  Unilateral osteotomies are then the likely answer, but your septum or tip complex may also be crooked.  That's why you need to see someone to discuss your issue fully.

It is possible to have a unilateral osteotomy if that's what's needed. You need to see a rhinoplasty surgeon for consultation and possible computer imaging. There may be other areas that need to be addressed that you don't appreciate yet in order to get the result you are looking for.

How did your nasal bone move?  The nasal bones are set and it would take some type of nasal trauma to collapse the nasal bone.  Osteotomy along the top and base of the nasal bone(s) would be required to shift the nasal bone.