Help Needed: Water Softener Draining Constantly

Hi all, I'm a little green on this stuff.
The water softener was unplugged for about two days. After plugging it back in, two things seem to happen. (1) there is a hissing coming from the control top, and (2) the tank is always draining a steady stream of discharge.
Any thoughts as to what the problem is, and whether or not I can fix it myself?
The brand is Osmonics Autotrol 255/460i
Many thanks in advance to anyone who can help.
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Sean,
You can find a manual with troubleshooting advice at http://www.pure-earth.com/PDF/255-460i.pdf . But first put the machine through a regenerate cycle and see if that clears up your problem. A regenerate cycle will take several hours.
Good luck, Dave M.
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Thank you.

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