Recently my compaq hp presario cd/dvd drive hasn't been able to detect when a cd is in the drive, it makes the usual sounds though on the computer it says nothing is there, though when i have my sims 2 dvd in the drive the icon does appear, what could be wrong, and how can i fix it?

Sounds like the lens is dirty, only thing to try is a disk cleaner, most supermarkets stock them these days, it is in the form of a CD, but it has brushes on it, apply any solution it comes with and follow the instructions, if your lucky it will do the job, if it's still not quite right try repeating the cleaning several times, with luck it will be enough.
But the reason one works and one doesn't is quite simple, there are 2 lenses, 1 for DVD and 1 for CD, now a way to help minimise this is to ensure your disks are clean before putting them in the drive, this helps reduce the chances of the situation your in now.

Now worst case and this doesn't work then your looking at a new drive, not too bad as they are so cheap now, certainly not ideal, but given the low cost, getting the drive repaired would cost more in time and labour than replacing it would, hope the cleaning does it for you though.