A 550 Error means "Requested action not taken. File unavailable, not found, not accessible." Since the file is available that means the server isn't accessible. That most likely means one of the following:
- You have used your Transfer Quota up, Tim Post suggusted this
- You have used up your server space Quota
- Your account has been locked by the hosting company due to some issue like not paying or a clerical mistake on their part (this happens)
- Your host's ftp server is having an authentication issue
- Someone hacked your account or the ftp server. I have seen this happen but they weren't able to change the ftp passwords.
Check with the hosting company to make sure you have not used up one of your Quotas. You should be able to do this through their website. If that isn't the case then give them a call and let them know what is happening. They will be able to check your account and see if it is locked or having some kind of issue. Usually they will even reset your password for you if that is the issue for some reason. If this isn't a quota issue then it is probably an issue on their site and contacting them is the fastest way to fix it.
Also, it would be helpful if you give us the name of your host. If you aren't hosting then you need to log in to your remote server and check on your ftp server software. It has most likely had an error.