Topband: Reactance problem

[Bill Gerhold]

Why would the reactance change on a ground mounted vertical
for no apparent reason.  I used to get a resonance point at
1.840mhz.  Lowest SWR was at 1.850.  All was fine until about
1 month ago.  Checking the antenna condition again, revealed
that a zero reactance point no longer existed although the 
lowest SWR did not change.  Checking the antenna, I found one
ground radial broken at the radial ring.  I had 50 radials now
I have 49.  Is it possible that this one radial is no longer 
allowing me to find a zero reactance point on the antenna?
Shouldn't I find a zero reactance (resonance) point somewhere?
Tuning my MFJ unit, from 1.7 to 4.0 mhz does not find a resonant
condition anywhere.  I can get close to zero, 10 uH, but no longer
zero.  Any ideas what's going on?

K2WH