Hello Pranit and welcome to Seven Forums.
It would help if you gave us your system specs. At the top of any Forum page you can click on "User CP". Then in the left column click on "Edit System Spec". If you don't know all the information a free tool called Speccy can give you those specs. Speccy - System Information - Free Download
Some of the basic things to try are:
1. A virus can cause your problem. Run a full scan with your own anti-malware suite. Then run a second full scan with the free Malwarebytes. (No anti-malware product is 100% effective 100% of the time. If there was such a thing we'd all be using it.) You may have to start your computer in Safe Mode with networking through Advanced Boot Options. Safe Mode
2. While in Safe Mode you could also run a system file checker scan from an elevated command prompt (option two) that will look for damaged or corrupt system files. If problems are found run the scan 3 times rebooting in between each scan. SFC /SCANNOW Command - System File Checker
3. If none of the above helps, these troubleshooting guides that were prepared by one of the Forum experts may help. It will take time to do all the diagnostic checks but the guides have helped many people. Troubleshooting Windows 7 Failure to Boot Troubleshooting Steps for Windows 7
4. It would also help to know what things you've already tried.