It could have been just jarred loose, there should be a small door on the case, usually on the side, sometimes on the bottom, if you take out the screw it will pop open. The drive is easy to remove and re-seat. If you can't locate it, check Dell's website for the manual for your PC model.
Hard drives are very sensitive on laptops. It can be easily fixed. first try to see if hard drive is working or not. as hand mentioned Dell website they offer only drivers, they do not offer boot cd. your dell laptop might be with Windows XP operating system. you need dell xp reinstall cd.
if your hard is damaged then you need to replace hard disk. you are looking at $40 plus dell xp reinstall cd costs around $20 and your laptop is back to life