Whether you have accidentally deleted some files or are in the "OMG I FORMATTED MY MEMORY CARD HELP" situation, this article aims to help. Note that currently this article has information
There are quite a few different ways you can accidentally delete or 'lose' files, ranging from pressing the delete button to not saving them when downloading from your e-mails. In some of these cases, the data will be gone forever. However, most of the time there will be a way to recover your data. In all cases however, you should (if possible) stop using that computer/ramstick/camera card (if computer, then use another to read this guide/ask for help and download relevant tools).
Ways you can lose files
The delete button
When you select a file and press 'delete' it will usually be sent to the recycle bin, and this is the first place to look for any files deleted this way. However, some usb sticks or camera card readers will directly delete the file without sending it to the recycle bin, or you might have emptied the recycle bin. If this is the case, skip down to Disk identification.
Very occasionally when you put a camera car/ramstick into a computer, you will be asked to format it. This means something has gone wrong. Sadly, some people compound the problem by not reading and just click "ok" -> "start" -> "ok" and may be somewhat surprised to find they have a blank disk waiting for them.
If you did not do this, you can skip down to 'Recovery using photorec' to start looking at how to recover your files. If you did do this, we need to know if you performed a 'quick format'. A quick format is performed when the 'quick format' tickbox is checked (surprisingly). If you can't remember, the defining characteristic of a quick format is that it is very quick (surprisingly) - if it formatted in a few seconds for ramsticks or camera cards/ under a minute for large hard drives, it was a quick format. Skip down to 'Recovery using photorec'. If it turns out it was a full format, you're out of luck - any data recovery is beyond the scope of this article.
"I pressed open on Internet Explorer (or other browser), now I cant find it"
In internet explorer this is a particular problem because it is very tempting just to press 'open' when downloading an attachment from an e-mail, instead of saving to a known location and then opening. This means that any subsequent saves of the file will be to temporary internet files and you will need to look in there for the file.
The paths vary between xp, vista and 7 (insert paths here).
Recovery tool walkthroughs
For deleted files, you need to identify your disk, as there is a chance it is ntfs - if this is the case, you can use the vastly easier to use undelete.
Recovery using photorec
Both ntfs and fat, a bit complicated. See this post - http://www.thestudentroom.co.uk/showthread.php?t=1349263&p=26724467 Instructions to be transferred to this wiki All formatting recovery attempts should be with this until ntfsundelete has been tested.
Recovery using ntfsundelete
NTFS disks only, i.e. not camera cards, not usb sticks. http://ntfsundelete.com/ Unknown whether it can recover files from formatted partitions.