And keep an eye on things like the Amazon deals of the day and Neweggs lightning deals or whatever, they have pretty good deals on SSD's and HDD's on a pretty regular basis. I agree with scribbles though and was just about to post, check your BIOS first and make sure the boot drive did not somehow get reassigned off your HDD, it is possible but somewhat unlikely unless you were in there messing around. Also, it could be a loose connection to the HDD, I would check the BIOS first, then figure out how to remove your HDD and check the connection. If that all looks good and you still cannot access the HDD, then I would look into a replacement.
If this is sudden it sounds like your HD went, might have to buy a new one. If you were messing around in the BIOS, you may have managed to change the boot drive to the optical drive, in which case you should switch em back.
You can do it yourself, but if it is still under warranty it could void it, they can be kind of picky about self-repair. If not, ask for help here or there are plenty of guides online that may even be specific to your laptop which will make your life much easier.
edit: You should check the BIOS to see if it lists your hard drive, and if it does, attempt to reinstall windows or attach the drive to another computer and try to get your data off of it.
Typically, there is a removable panel on the bottom of the laptop (4 or 5 screws holding it in place) for the hard drive, and the drive pulls out pretty easily by pulling it way from the connector, then up out of the laptop.
You can replace it with any new 2.5" SATA hard drive (get the size you want). SATA 2 or 3 are both ok to use, and will run the same speed. Getting a 7200RPM drive can make the laptop a little zippier (assuming you didn't have a 7200RPM before).
If you don't need much storage space, an SSD hard drive will make the computer MUCH faster; a 128GB drive will cost about 100$ give or take a bit, and they go up in price from there.
If you want any advice on what hard drive or SSD to buy, just post back.