Disappearing Channels

[milli260876]

I've had this issue once or twice before and can't fathom it at all...
this time ive lost channel 4hd. Its just gone, if i look in channel edit nothing. If i try to assign a channel to that number(4) it won't work???
anyone got any tips/ideas?? I'm loathe to set tv up again. I've tried scanning the transponder to no avail.
i'm contemplating tvsource as guidetool has never worked well enough for me on back ups and the number of new transponders makes set up a PITA.