Could either be the Glow plug relay or 1 or 2 glowplugs gone.

Disconnect leads on the glowplugs first and check for continuity on each one.

If you don't have a meter or don't understand the above, disconnect each glowplug, connect a wire to +ve on the battery and flash the lead to each terminal on the glowplug, if it sparks it's O.K if it doesn't replace for new.

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