I cannot upload files

Since the server crashed on Friday, I’ve been having this problem with my index.php file. When I checked, I saw that the file was blank, even though it was working correctly with the code that was saved. But when the server crashed, the file’s code was completely erased. Now, when I try to recreate or upload it, it won’t let me. I keep getting the message “Failed to create file: Failed to create file on FTP server.” The file is 4kb, and when I upload it, I get this message: “Upload failed: Failed to upload file: index.php”