I used the loop cut tool around the side of this model and then dissolved the edges because I thought it would save on file size and rendering and now I cannot select the area marked with the darker orange lines at all. Is this retrievable or will I have to start again?
Those dark orange edges are on a separate object which is selected outside of edit mode. Either you had both objects selected when you went into edit mode, or you Ctrl + xlicked it while in edit mode, or (more likely) you inadvertently disconnected it by hitting P with that geometry selected.
Either way, you can fix this by exiting edit mode and joining the two back together again with Ctrl+J (with both still selected).
If you need to reconnect the edges, select the vertices in pairs and merge them with Alt+M. (Selecting all with A and hitting W > Remove Doubles may also work to reconnect at least some of the geometry.)
I have had a mesh become corrupted so that certain functions/actions cannot be performed on it. If it seems your mesh is corrupted, which it may be, try various creative ways to trick the software, such as:
- Export an .OBJ, and then open/import the file into a new blender project.
- Use a mirror modifier or array modifier to create a pseudo-copy. Apply the modifier and then delete the original, leaving one of the copies. (Mirror back if needed).
- If these don't work, you still don't need to start completely over. You can quickly rebuild it using the corrupt mesh as like a template that you can snap the points of the new mesh to the points on the old mesh.