how to fix my g520 laptop?

Try starting in safe mode. Tap the F8 key a few times just as the POST screen disapears. It should bring up the start options screen.
Some times, just starting once in safe mode will be enough to fix the problem.

If you can't start in safe mode, or starting in safe mode was not enough to correct the problem, try to restore to a previous state. Also from the start options screen.

If restoring to any previous state did not work or failed, you have a more serious problem. Your Windows instalation is severly corrupted and you need to reinstall.
Get a Linux live media. Linux is a free and open operating system.
Boot your computer from that media. Enter the trial version. It don't write anything on your disk: It works entirely from the DVD or USB medium used.
Use that to copy all your used data to soem external drive.
Reinstall Windows. If you are offered the option of "Update", use it. That option is safe for your data, user settings and preserve your drivers and installed applications.

If the update option was not available, reinstall all your drivers, all your applications.
Restore your saved data.

Let Windows updates do it's job getting and installing all available updates.

Electro
October 2013