As this is clearly for a diving application.
1. First we need to know the size of the compressor or specific model number.
2. Did you buy it new yourself or second hand if so we need to know the wording on the motor data plate.
3. Can you verify if it was working in the same location with the same wire harness, starter etc two years ago.
This just get all up to speed with a known base of background and information.
Saves us asking did you dick about with it.
I
It has been standing still the last 2 years.
Again for all to get a better picture.
4. Did you own the compressor for the two years it was idle.
5. Was it drained of oil or inhibited (bagged and sealed) for this storage period.
I
Now I wanted to start it up.
Nothing.
Only a humming sound.
Are there things I can try before calling a specialist
I agree with every thing said by others so far but would add some caution for you not
to cause any serious damage trying to re start this compressor after a two year idle period.
If it were me I would remove the drive belt and separate your planned maintenance between the
electrical, wiring, starter box and motor, and separate from the expensive to fix compressor block.
Once separated by simply removing the rubber drive belt (or belts depending on model) you cannot damage the compressor block. while you can run the motor (even for hours) to get that running correct while the block is safe from any further damage.
So we now have two separated major components and one cannot damage the other.
So please dont try and start it again without first removing the drive belt. would be my first advice. Iain