Time machine stopped working unexpectedly

Was working great for a while. Then it stopped. I think that was because of the server computer- it just couldn't read the drive. Then it worked again. Now it has stopped again with an error saying that the drive is not writable. I don't know what this is about- since I spent all that time making it writable and then it worked for like 5 months. Haven't had time to work on this because I've been too busy writing my dissertation which really really needs to be regularly backed up.