Each webhosting account has a disk quota - a limit on how much disk space it can use to store files on the webserver. On older sites this can be as low as 5 megabytes: a new one would come with 50 or 100 megabytes.

When the disk quota is full, your FTP program will give errors when you try to save new files. On most webservers, the web applications will also fail to write files. If this is happening, the quickest thing is to email webmaster and ask them to check.

Older sites can often have more space, just for the asking: they're welcome to the same quota that a new site has. If a new site runs out of space, there is often more available at a cost. Adding 50 megabytes to the quota adds $20 per year to the hosting.