Hi there mick - welcome to RadioGeek!
Righto - first thing to do -
don't get desperate ! Most often, data recovery is prevented by people trying to get data off themselves in the incorrect manner.
I am assuming that:
- You have tried disconnecting and reconnecting the drive and confirmed that the cables are correctly connected at both ends
- you have tried using different cables - I have found that damaged drives are often just damaged cables
- you have tried your drive in a different computer - this rules out the issue being a faulty bios/motherboard/controller
- this disk was previously working correctly on this machine & operating system
If you haven't tried the above - you may want to do that first.... if you have, read on!
Next you need to decide a few things. How important is your data? The FBI can recover data from a hard drive from a computer out of a plane crash in the bottom of the ocean - but for you that kind of technology is available - but very, very expensive. So, a question for you - how important is your data?
There are several things you can try yourself - again the first tip applies. Unfortunately, tricksters on the internet know that people are usually desperate and will use their credit card to buy software that will "guaranteed recover data", often when they would never dream of being taken in by this normally. Most of your effective recovery tools are available online and free if you know how to use them. Short answer here,
- don't start buying stuff that promises to solve your problems
- don't download and install free stuff that promises to sole your problems
If it sounds too good to be true - it probably is.Data recover is a very specialised and technical field. If your computer is not working when you try to boot your machine, then it could be that a hardware fault on the drive itself is causing the problem. There are many techniques - I have successfully recovered data from a faulty drive by replacing the circuit board with those from an identical drive - but these are specialised and rare cases.
If you want to, there are LOTS of forums out there specialising in this problem. Try googling up your own, for example:
http://www.datarecoveryforums.com/http://forums.getdata.com/computer-data-recovery/Couple of tips again - Read LOTS, and judge your advice and forums critically. Those that only have a few posts and members are less likely to be legit.
Lastly - data recovery is really hard to get right. If you don't know what you are doing, and if the data is super important to you - google up some experts in your area. There are many many companies offering data recovery services - get some expert advice!
I am happy to take a look at anything interesting that you find if you want to post a link here and offer you a second opinion

Cheers, RadioGeek.