Here's a picture of the M25 decompression lever. The normal (compression) position of the lever is shown (lever to stbd / no spring tension). Lever to port (full spring tension) is for decompression.
The lever should NEVER be moved while the engine is running. It is typically only used for starting (cranking) a very cold motor or when battery voltage is marginal for cranking.
The lever should only be held in the full closed (compression) or fully open (decompression) position, again, with the latter only used for cranking. The lever is not used for any in-between positions for adjusting the valves.
The spring holds the lever in the full closed (compression) position. If your spring is broken, I would just wrap a small wire or zip tie to keep the lever in the compression position until you get the spring replaced. The spring is not required in the "compression" position so you could just leave the lever wired closed. The spring is only used when cranking a cold engine, so when the engine is cranked up to speed (with no compression), the lever quickly snaps back into position when released, allowing the engine to start.
For your particular non-start issue: Try the glow plugs. At my first launch after getting the boat, it took me two hours to get the motor started. I was holding the glow plugs 15-20 sec as I had read somewhere. With the old, original wiring setup I had at the time, it actually took more like 30-40 sec to get a cold engine started. Since rewiring the circuit and installing a glow plug relay this season, I now use 10-15 sec glow time. But, I find that less than 10 sec = weak start up (multiple engine turn-overs before starting) while 15 sec = engine immediately roars to life.