From accidently deleting a file to a complete computer crash, providing your hard disk is still functioning, most data is easy and cheap to recover. On the rare occasions that a hard disk has stopped working completely, it becomes a bit more challenging, but don't worry, there are things that can be done.
If you have deleted a file when you weren't supposed to, you haven't really deleted it. The system just removes the entry in the file index, it doesn't actually erase the file so your data can be recovered with the correct utilities. You must switch off your computer immediately though. If you continue using it, you run the risk of overwriting the file that is now marked as clear space. Use another computer to search for a 'file undelete utility' on your favourite search engine, you should be able to undelete the file yourself, but you must do it without starting your operating system to maximise your chances of success (e.g. by starting your computer with a boot CD or putting the hard disk in another computer). If you're not sure, get in touch as this kind of recovery wouldn't normally cost more than £20.
If your computer or portable hard drive is not starting, it may be that the hard disk is faulty but it's more likely to be an operating system file corruption or an electrical problem. If this is the case, the data can be retrieved easily and is also inexpensive to do, depending really on how much data you have got. Get in touch and I'll be able to give you an idea of the cost.
If a hard disk is failing, you normally hear tell tale signs before it stops completely. If you hear any grating or grinding noises, or maybe an 'electronic ping'-type noise, then switch off your computer. You will increase the chance of data recovery by not using it and compounding any damage. Low-level recovery utilities can then be used to try and salvage any data.
If an electrical component on the hard disk has failed, these can be repaired, but it requires specialist tooling and becomes more expensive. Again don't worry, most data can be recovered, it is only on very rare occasions that a disk is destroyed beyond repair.
Remember, the first thing you must do is stop using your computer - switch it off completely! This prevents the system from overwriting any data or the problem getting worse. The second thing to do is stop kicking yourself for not doing regular backups (you're are not alone). Get in touch via the contact page - 9 times out of 10, recovery is normally a straightforward process, and shouldn't cost you hundreds of pounds.