Tutorial: How to fix Mac file problems ~ Share Hacker

What can you do when all of a sudden one of your apps no longer opens?
Have you checked it's the latest version of the app? If not, update it.
If it is, open your hard drive and go to Library/Preferences and find any .plist files related to that app. Drag them to your desktop. Do the same for the Users/youruser name/Library/Preferences folder.
If not, uninstall it. Don't just drag its icon to the Trash, but run its uninstaller or use a program such as AppDelete or CleanMyMac. Then restart your Mac and reinstall the app.
If not, it could be conflicting with something else on your Mac. So, back everything up, wipe your hard drive and reinstall OS X. Install the app again. If all's well, don't use Migration Assistant because this could reintroduce the problem - instead, reinstall your apps and copy back files manually from the backup.
"I've got a file I can't open! What can I do?"
Open with
You're probably missing the app you need to view it. Does it have .docx or .wmv on the end of its name? This will help you pinpoint what kind of file it is, and therefore, what software you'll need to open it.
Do a search on the web for the file extension followed by the word 'Mac' to find an app to open it. For media files, VLC is a good bet.
If you think one of your existing apps should be able to open the file, drag it onto the app's icon (either in your Dock or the Applications folder). This will attempt to open the document using your chosen app. If the app doesn't go dark, press Command+Option to force it to try.
"A file I've been working on is now refusing to open!"
What error message are you seeing? If it's that it doesn't know what app to open the file with, right-click the icon, hover over Open With and see if you can use one of the apps there.
If you don't think any of these will do the trick, or the app you want isn't listed there, click Other… to browse your Applications folder to find the right one. If you can't find the app, it may have been deleted, so pop in the installer disc, or download it again.
The other possible error message is that a particular app is unable to open the file. If the wrong app is loading up, follow the above instructions to open it in the right program. But say the correct program is unable to load it - then the file is probably corrupted.
If so, the best option is to delve into your Time Machine backup and restore an earlier version.
"Spotlight can't find my file - but I know it exists!"
Open System Preferences and click Spotlight. In the Privacy tab, make sure the folder containing the file isn't in this exclusion list. If it is, select it and click the '-'.
Another cause of Spotlight failing to find files is that its database needs rebuilding. Drag your hard drive icon into the exclusion list, quit System Preferences. Then open it up, go back to the pane and select your hard drive. Click the '-' and Spotlight will re-index your disk.
How to recover accidentally deleted files

Have you got a backup, Time Machine or otherwise? If so, recover the file from there (open Time Machine, browse back to the file and click Restore). Some apps include auto-save and autorecovery features, which may be able to salvage something.
Otherwise, you may be able to get your files back using special recovery software such as Prosoft's Data Rescue 3. To give the recovery app the best chance of success, run it as soon as possible after deleting the file.
Data Rescue 3 will scan for free and allow you to recover a single file under 10MB, but for the full functionality, it will set you back $100 (just over £60). You'll need to assess how much that lost file is worth.
Quick tip
If you don't already use Time Machine to make regular backups of your Mac, go out and get yourself an external hard drive now!
Keeping backups will mean you can recover most things, should something go amiss. Set up Time Machine in System Preferences > Time Machine.