Tricky........
Are you sure that you have not accidently switched to mode where your laptop is only displaying to external monitor. you should find instructions in handbook. See if both external monitor and Main screen show up on display properties, make sure you know how to switch back to your external monitor if it does not automatically find and display on external monitor first.
if the default monitor montor setting is not the problem then your screen has failed and you will need to have your screen replaced. if you still have warranty then send it back to manufacturer or where you purchaced it from.
No warranty? then take to local computer shop, (shop around dont just go to PC World or major outlet)
Laptop displays are very expensive.