Bad battery, bad solenoid, bad cables.
Check your alternator and your connections form the battery to the Starter. You may just have a bad connection If that does not work, your solenoid on your starter has gone bad, a new solenoid or starter may be needed. Check the start switch on the clutch pedal if so equiped.
You have a bad starter solenoid or for some reason starter is still not getting enough current-which could be the bat or connections are corroded etc. Check bat voltage and connections (all) to starter.
most likely tha starter is bad or the power to the starter is bad
Yes, most likely its the starter if the battery is fully charged and it still wont crank. When you turn the key, you should hear a click. That click is the starter engaging. No click, means the starter is no good.
Either your starter solenoid, if not check the neutral safety or clutch safety switch.
Check your electrical connections
Most commonly, when you get a single click when attempting to start the engine, the starter solenoid or relay is faulty.
If you are 100% sure the battery and ignition switch are good and it still will not turn over, suspect the starter or starter solenoid. If you hear it click it is getting power. The only 100% sure way is to remove it and have it bench tested at an Auto Parts Store.
Do you have any click in the starter itself but still not turning over? Then its the starter giving you fits. Replace it. If no click is heard then it could be a fuse or more likely the starter relay. Check them both before moving to the starter.
Check the battery, battery cables, keyswitch, neutral lockout switch... your old starter may be just fine (sorry to say) or the new starter may have a problem. There are too many unknowns to be able to tell you anything definitive.
The problem may be with your starter and not with the clutch.
Double check the battery cables at the battery and at the engine, be sure they are clean and tight.