If Time Machine on your Mac can’t find your backup disk
Try any of these suggestions if Time Machine can’t find your backup disk.
If your backup disk is connected to your Mac, make sure the disk is firmly connected, plugged into a power source, and turned on.
If your backup disk is connected to a network, make sure your Mac and disk are still on the same network, the disk is turned on, and the network isn’t experiencing problems.
If your backup disk is connected to another computer, make sure the computer isn’t in sleep mode. If it is, disable the sleep function for future backups.
Open Time Machine settings and reselect your backup disk. If you erased your backup disk and began having trouble, reselecting it may fix the problem.
If you’re using a Time Capsule as your backup disk, use AirPort Utility to make sure it’s set up properly and connected to your network.
Click the Finder icon in the Dock to open a Finder window, then look in the Locations section. Your backup disk should be listed. If it’s not, it may be turned off, defective, or currently unavailable on the network.
If you back up to a Time Capsule or other network disk that has a password, the password may have changed. Log in to your Mac as an administrator, open Time Machine settings, reselect the backup disk, then enter the new password.
To open Time Machine settings, choose Apple menu 
.