Its easy, 1) Make sure you have a Bootable Windows 10 Pendrive, if not the make one, its easy, youtube has many videos on it. 2) Just buy any 2.5 SSD and replace it with the existing HDD. 3) Insert the Bootable Pendrive and boot the laptop and follow on screen instructions and instal Windows 10. ALL DONE
Hey, it looks like it might be your motherboard or hard disk issue, if it doesnt boot into ur hard disk then ur hard disk is gone, bt u say it does boot at times then my guess would be to get your motherboard checked as it might be an issue with your motherboard (could be your motherboard IC).