Need help quickly : Can't upload any files!

I keep getting this message:

Error, cannot overwrite remote file.

Permission denied. Error code: 3
Request code: 3

What does this mean?

I have never had this kind of problem before.

A lot of my files has gone missing now from the web and I really want to upload them again as soon as possible but at this moment I can't do anything. Not upload files, not delete files.

It means what it says. Either you do not have enough permissions to modify the files themselves, or the directory, where the files are located.