Suddenly created meshes are invisible (even though they are checked as visible), cannot create any new objects now as the title states I am suddenly unable to create any new objects, when i select a mesh from the "create" bar nothing noticeably happens, other than the object name appearing in the list in the top right hand corner of the screen, i cannot edit the object, view the object, or do anything at all to it, this happened suddenly part way through a project and has caused me to be unable to continue, it's driving me nuts and I cannot for the life of me see why this has suddenly happened, im sure i must have accidentally pressed a keyboard shortcut for something, but i do not know what, any ideas?
You probably activated Local View by pressing Numpad/.

When that happens you can only see and edit the selected object.

Press Numpad/ again to exit (or press the Space Bar and type Local view)