My laptop Dell Vostro 3450 is always run to BIOS when I start the laptop

Please help me on this issue. I try so much to start the window but my laptop is always run to BIOS options when I start laptop. 

Sometimes, it is run to Window 7 Ultimate, but recently, it is no longer go to Window.

My laptop is Vostro 3450, RAM 2GB, HDD 500GB & Intel HD graphic.

1. When I change the options in BIOS for HDD in Advance tab from AHCI to ATA, then I changed it again from ATA to AHCI. Next, it will run to Window, but recently, it is not displayed window anymore.

2. After that, I try to use the option Diagnotics in the BIOS setup screen. It is run from the first option to HDD option & it stop at it forever, I must press the button to turn off the laptop. So I thought my HDD is bad, I must change it, right? Please kindly help me.

3. Yesterday, I bought a Hiren's BOOT CD and put it in CD-ROM, then I open my laptop & press Start button. It is run to Window, I am so happy. But after that, I turned off my laptop & It is always run to BIOS again.

Therefore, now, I can not use my laptop, please kindly help me to fix this error. I have a lot of important data in my HDD. Please kindly help me, I cannot bring my laptop to Dell store to fix again because the expiration date  have ended at 11/2013. 

Please kindly help me. 

Thanks in advance,

kpopyo