bro i also facing this issue in my old hp laptop & go to service center, service centre person will clean the laptop cooling fan & apply heatsink thermal paste:k
@Arjun3144 can you first share the basic specs of your laptop... and the model number...
Consider the steps, whichever applicable to you, i just listed all possible ones...
If not booting at all...
Does it Turn ON - Does any of the LEDs glow when you power ON the laptop
If it turns ON, does it boot - Once turned ON, does it boot to windows
If its booting, does it go to your desktop or restarts in between the boot and go to black screen
If it doesnt boot to your desktop, use recovery discs as stated by wewake
Also i believe Lenovo comes with "One Key Recovery", use that if available.
If it boots...
If it boots to your desktop, does it automatically go to black screen after few minutes.
Is this happening in a specific application or randomly or even when idle.
Install Core Temp to check changes in temp. of the processor
Check task manager for sudden spike in RAM and CPU usage
If the temp., RAM and CPU usage increase rapidly, Check which program/service is causing the spike using, Start >> Accessories >> System Tools >> Resource Monitor
Do you have windows update turned on? If yes, did this happen after a update. If yes, do a system restore and check if it helps.
Boot to safe mode, and check for duplicate/wrong display drivers.
Connect another monitor and check if it works correctly.