These red line appeared over every path in the design I was working on and I cant seem to get rid of it, it even appears when the image in exported. I know it must have been caused by a button I pressed by accident but cant seem to work out what by googling the problem. Changing the path color just changes the color under the red lines. Would love some help with this! Thanks!
You probably applied an (outline?) effect to all the shapes, try going to the Appearance Panel and deleting any unwanted effects.
Window → Appearance or Shift+F6
Select the un-desired effect and Delete
Since this is altering everything on one layer, the stroke may be applied to the Layer itself.
Highlight the layer in the Layers Panel, then check the Appearance Panel for a stroke and remove it.