Carver Cascade 2 water heater not working....how do I fix it?

Using on gas you should get:
Green as you have which says all is ok burner has lit, allow about 45mins for heating up.

Green and Red means ignition failed either through gas module valve not opening or gas bottle empty, in any case of failure to light allow 2mins before trying again to allow any residue gas to clear, gas bottle empty. Amber means low battery.

As your green light is on and if you have waited 45mins and still no hot water then get a Gas Safe Engineer who is certified to work on LPG systems to check it (UK law covers leisure gas/electric appliances. If its a duel fuel (Gas and Elect) you will have another switch for use on 240v hook up shut off the switch for gas operation turn on switch for elec which is same as home switch with red neon on indicator light. You will know it got the power unit on the end of the tank usually under a white plastic cover (under which is also reset button if on elec over heats this will trip out press to reset but find out why if it trips again) with blue cable coming from it to the switch. Can't help any more good luck.

Bob
October 2014