Most likely, you changed the format from 1080i to 720p....or something like that.
The simplest fix is to do what the guys above said: connect the nfusion to an old TV, using the RCA A/V jacks (yellow, white, red), although it's really only necessary to hook up the yellow one. Then, you can see the display well enough to go back to the menu and change back to the format that your HD TV wants (1080i or 720p, and NTSC.
I that doesn't work, then there's plan B:
I have a nfusion HD, and here is how you can change the AV output setting:
Turn on the nfusion
Wait about 20 seconds for it to boot up
press "exit" a few times
press "menu" once
press the down button once
press "OK"
press the down button once
press "OK"
Press the "right" button once and wait about ten seconds (This toggles the "TV Standard" between NTSC and PAL and MULTI
This will probably fix the problem, but if it doesn't then:
picking up from where you left off, press the down down button twice
press the "right" button once and wait about ten seconds (This toggles the Format between 1080i, 720p, and a couple more formats.
Good luck