OK, first of all, a signature will just not "disappear".
If it breaks a limit, it will tell you, and let you correct it. It will NOT just delete anything.
My guess would be that you uploaded a new signature and didn't read the huge red bold letters that tell you that your previous image has been deleted.
With all due respect, i hadn't touched mine after the new rules were set until i noticed mine was missing. so i checked it to see what was up. it was in the signiture spot, but it wasn't showing up. Everyone elses was though. So then i tried to put it back on by starting over which screwed me over big time. But no sweat BX is making me one, so it's all good.