Strange behavior of a DNA gel
If you use pre-staining, the stain migrates as well in the electric field and this can lead to some parts of the gel being unstained. This would look pretty much like you describe, the ladder being invisible as well points strongly to that part of the gel just being no stained properly.

It sounds a bit unlikely with your running parameters, I've only seen that with a very long running gel, but I also haven't used pre-staining for quite a while now. Ethidium bromide is positively charged and migrates in the opposite direction as the DNA, I have no experience with Sybr-Safe though and don't know if it behaves the same as EtBr in this regard.