Hp laptop black scree after Windows

Hello,
Good morning, please i need help with my HP Laptop as when i power it on, it comes up plus the windows display on the screen but after that it stays blank and brings nothing. I do not know the cause of that but i thought it could be the hard drive, so i went ahead to change the head drive i.e fixing another hard drive on it and it eventually loads, bringing in and displaying..working perfectly. so with this update the previous hard drive was placed back and it did not come up just like the other time. and i have alot on it that i do not want to lose. Please i need your guide on what to do without losing all my files.