I guess you're on the Chopin server? That server is currently full in actual hard drive space. Until the server is upgraded you wont be able to upload any more files. You can check out it's status here http://status.x10hosting.com/
Go onto your cPanel. You can then find out which server you're on via expanding your stats on the left side, it will show it beside Server Name. Or you can look at your cPanel's web address. It will be SERVERNAME.x10hosting.com:etc.