What could make a positive hiv result look negative? i was dehydrated and had the chicken pox could this have made a difference?

Unlikely: Neither dehydration nor another viral infection such as chicken pox should interfere with testing for hiv. What kind of test was done? I'm assuming you had a definite positive previously and are now negative--what treatment have you been getting? Some treatments work so well for some people that viral load is vastly reduced, which might be called 'negative' under some circumstances.