User account settings will not load ???

[Gmac61]

I'm relatively new to macs, so be gentle.......
Help, please ? For some unknown reason all my settings have reverted to default including my email and browser........ losing addresses and bookmarks (Firefox & Thunderbird).
No reason I can find, but my disk space is the same and all the files are still there...... could be something to do with User Accounts - although there really is only one....
In the Users folder, my original desktop and settings seem to be there including all my saved files (I can see the different icons etc) but I cannot use the applications with my original settings- all the applications seem to have reverted to default and I cannot log in and see the desktop I had and the settings. Again, it seems to be there, because I can see it under "Macca"(my User name) in Users? There is another name there from the previous owner (which is somehow the "Short Name" in user Accounts in System Preferences), but no stored files, just default stuff. There are no other users in User Accounts in System Preferences.
Forgive my bad explanation, but this has got me stuffed and I'm desperate because it will mean setting everything up again !! And I cannot retrieve things like my address book.....
Any ideas ? Jeez i wish there was a restore setting to get me back to where I was this afternoon!! Aarrggggg.......