Did you try and do a hard reset holding down both reset and power? you may just have to keep messing with it by connecting to your comp. I have had that happen before and just kept playing with it til it came back past splash.
Thanks, yeah tried the hold down trick, at some point with a bit of luck it appeared in tomtom home, so was able to eventually get it back, i suspect map file didn't transfer correctly, deleted map, tried to start and it started normally but reported no map!
put map on and cracked, and all was well!