can't backup with time machine

I can't backup with time machine and I do not receive any errors, time machine just stays in the 'looking for backup disk' mode and will not go any further. I went as far as buying a new usb backup disk to replace the old one and still the same issue occurs. I've tried using different usb ports on my mb pro and that doesn't work. No amount of restarting, ejecting discs or checking preferences helps. Disk utility states the drive is fine (even the new one), the permissions or sharing haven't changed.

My system is up to date with the latest updates, and this all just happened out of the blue one day. I suppose I could do a system restore but I'm afraid of losing my data; and I'm not 100% sure how to do that.

I'm running OSX 10.8.2, 2.3 GHz Intel Core i5, 4GB memory