Computer is not recognizing DVD-Roms?!?

Ok, so I have Dell Windows Vista, and I got it in 2007. For the past 2 years, everything has been fine, mostly. Just small things.

But for the past like 2 monthishs, I have been trying to install Dvd-roms. And as said in my title, my computer does not recognize them. And they have for the past 2 years.

I do not know what i did. But its pissing me off. My sister has the same exact computer as me, got it the same year, same time, etc. and they work on hers.

Is there anything that I could do? I do not want to hire a professional because 1) I do not have the money right now, 2) My parents do not have the money right now to waste on my computer, 3) I want to first know if i can fix it without a pro.

Can anyone help me?

*Note: Its just dvd-roms, because cd-roms work fine.