First, restart your Mac in "Safe Mode", followed by restarting normally. Determine if that fixes it.
If that does not work, try "reconnecting" to the Time Machine backup disk:
Open Time Machine Preferences and de-select the Back Up Automatically checkbox. Make sure no backups are in progress, otherwise you'll have to wait for it to finish or terminate.
Then, click the "Select Disk..." button, select the backup disk, then Remove Disk, then Stop Using This Disk.
That will not affect any existing backups.
Then, Select Disk... again. Re-select the same one under Available Disks, then Use Disk. If the backup is on a network (TC or AEBS), you will be prompted for the password you provided for that disk in AirPort Utility.
"Waiting to complete first backup" will appear, which means it won't do anything until the next scheduled backup. That does not mean it needs to create a completely new backup, as if it's creating a brand new one. Your existing backups will still be available.
If you don't want to wait that long, select Back Up Now.
A Notification may appear if your source volume is encrypted and the backup volume is not. That's normal.
Re-select Back Up Automatically. The "Preparing Backup..." status message will remain for a long time. Eventually, "Backing up xxx of yyy" will appear, but those values will not be accurate, nor will the "Estimated time remaining" in Time Machine's Preferences. Just ignore it. It may take a few hours even if your Mac is not allowed to sleep. Time Machine will also pause or become slow if you use your Mac for tasks it considers a higher priority, which is new in Sierra.
