Connect you external monitor and check whether you change any display settings in your laptop which shows only external monitor..
Also try this one.
Step1. Remove battery and remove power supply
Step 2. Hold down power button without power supply for about 1min
Step3. Plug in you power cable and push you power botton
Screen should come on at this point
Step 4. Put back your battery and continue
If this doesn't work, maybe your screen is bad or your connectors is having problem