If you can see fine on the external monitor, then your attached display, or the ribbon cable that connects it, are your problem.
If it is under warranty, then you need to contact the manufacturer about a repair. Now if it isn't under warranty, you can either try doing the work yourself, start with checking the cable for problems or being loose at either end. Or you can take it to a local tech.
Also, if the external monitor is using a different GPU than the main display uses, then it also could be the GPU that is the problem.