Time Machine won't back up.
I have Time Machine on my Mac and for some reason it has stopped backing up to my Seagate 2TB external HD. My son installed Time Machine but he is now overseas and I don't know what to do. I have used Disk Utility First Aid and there is no problems with the Drive. Can I reinstall the program and start afresh. Can any one help me. I am running OS X El Capitan 10.11.3.
Thankyou
Chris
iMac (21.5-inch Mid 2010), OS X Mountain Lion (10.8.3)