First of all is it regenerating? is the display showing the time of day? if it's showing a 2 0 then the shear pin is broken and needs replaced along with the orings on the piston.
are the little dots on the bottom of the display blinking? replace the battery.
if it is a steady alarm, it lost it's setting, see above, and reprogram by starting at the top red button on the left. press the red button, enter data on the white keys,and press blue enter. continue down through the red buttons and when you get done the time should show on the display.
is the triguard( plastic cylinder on the back of the valve that the brine tank attaches to) full of water all the time(you can tell by the position of the ball inside, it floats) ? clean the poppet(little white thing) under the triguard. there is pressure under the triguard, so bypass and relieve pressure before removing. If this doesn't work , replace the poppet and triguard.
If the triguard isn't full of water normally , and the ball is close to the bottom, is the brine tank close to empty? It should be. if not the problem could be a clogged injector,worn orings on the injector, worn orings on the piston, clogged screen on the bottom of the head, clogged distributor on the bottom of the riser pipe,bad drain tab, clogged flow control, plugged drain, clogged 1/4 gpm control for brine fill, clogged screen in the brine refill seat, bad tab on the brine refill seat. check all of the above, and let me know if you need more suggestions.