Jim
For what it's worth here's my two pennorth.
If the engine rotates when you turn the key but doesn't start, could be a pre-heater relay problem.
If you hear the 'clicking'/'buzzing' when you turn the key but the starter doesn't engage and the engine doesn't turn over could be a sick starter solenoid or just a bad earth connection.
First point of call for me would be to clean the battery connections.
If that doesn't work try shorting out the starter power supply connections with a stout screwdriver - beware there will be some sparks.
If the engine turns over, then the solenoid is not engaging the starter gear with the flywheel and providing the starter with power and needs some attention.
Regards CL