Time capsule not backing up

I got an error message saying that the computer hadn't been backed up in 13 days, but in the time machine, it says my last backup was done yesterday at 8:41 am, but it hasn't backed up since then. I can't even manually back up the computer, by pressing Back Up Now. When I do, it says preparing, then I get a message that says backup disk ejected. I haven't made any changes - the only thing I did was restore the computer from a backup, but that was over 2 weeks ago. I also tried restarting the computer and the time capsule. Any ideas?