Time Machine Hangs with Black Screen

Happy ex MS windoze user of 15+ here and very happy with my macbook pro - it's awesome. however about a week ago, even though time machine says it's working fine - when I enter time macchine I get a big black screen - nothing else - seems to hand there and only option is to kill app - any ideas anyone? I'm backing up to a 1tb western digtial USB drive, had been working fine - thinking I'll just reformat and try again?

Thanks in advance

P.

Have you been into Sys Prefs > Time Machine > Select Disk to see if the Ext HD is shown as available - you might need to reselect it.