Need to know what buttons your topside control has... depending on the model, some have a mode button which you press and release and then immediately press the temp up or down, to select SL for sleep (no heat), ST for standard (heat as needed) or EC for Economy (only heat during filtration). If the mode button is not present, and you have a single temp button and light button, try pressing and releasing temp and then immediately pressing light button, repeat until choice appears, then wait, it will hold last displayed mode.

SLeep mode. Exiting sleep is unit specific. A simple way out if sleep mode is to power cycle the unit by turning off power at the disconnect, wait a couple minutes and power back on.