Interesting ideas above...all probably wrong though!
The description sounds to me to be a beeping sound - a POST code. The vast majority of computers will not display anything on the screen if an error is discovered extremely early in the Power On Self Test (POST) and the frequency of the beeps is an indication of the error the computer has found.
If it's continually beeping a single tone each of identical length then most likely it's a memory error. One of the memory sticks could have died so you'll need to open up your computer and...
1) Remove all sticks of memory then reinsert them. Try the computer. If this works then it's sorted...but if not...
2) Remove all sticks of memory then replace only one. Try the computer. If this works then you know you have a bad stick...if not...
2a) Remove the stick that's in and try another one. If the computer works then you know you have a bad stick...
If you only have one stick then you'll have to get another if step one doesn't work. You could buy another as it's pretty cheap or borrow one from a friends system.
BTW, removing and reinserting memory is one of the easiest things to do inside a computer! 5 minutes, tops!
Another likely cause could be that your graphics card (if you have one) may have become unseated in it's slot. Again, this is easy to sort out - just take it out and put it back in!
If you don't have a screen displayed then 9 times out of 10 it won't be your hard-drive.
Have a look at
http://www.amptron.com/html/bios.beepcodes.html and see if that describes what's happening.
