First, I suggest you make another full backup to a locally-attached external hard drive. You should do that anyway. One backup isn't enough to be safe, and backing up over a network is less reliable than backing up locally. Then try each of the following steps that you haven't already taken.
1. Restart the backup device. You can do that by disconnecting and reconnecting the power cord.
2. Open the Time Machine pane in System Preferences and check that the device is selected as a destination. If it isn't, select it and try again.
3. Hold down the option key and select Verify Backups from the TM menu in the menu bar (not the Dock icon.) This operation may take a long time. If the menu-bar icon (a clock that runs backwards) isn't showing, check Show Time Machine in menu bar in the preferences.
4. Erase the backup device. With a Time Capsule, you do that from within the AirPort Utility application; see its built-in help for details. All backups on the device will be lost, so don't do this until you've backed up to another device. After erasing, select the device again as a backup destination in the TM preference pane. The first backup should be made over a wired connection, if possible.