I've had the same issue, it was the battery in the end..
My bikes a 1998 model, so I am currently in the process of changing everything out for new stuff, such as the regulator, starter solenoid etc.. but this stumped me due to my battery (gel type) being on a Noco maintainer. It was after the cold snap we just had, I went in the shed to get my bike started up and just got the click from the starter solenoid. I've had similar issues previously after cold snaps, but usually it is fixed by a wiggle of a few wires, and a spray of wd40. Not this time! So checked everything using a multimeter, the battery wasn't fully charging through the Noco, and was showing just above 12v, which should be enough you would think to turn the bike over. Checked all the system to the starter solenoid, similar voltage all the way... Gave the battery a full charge up over night, thinking it may just be down due to the weather, but all it did the next day was a couple of cranks and then ran out of juice.
I tested how many volts it was getting on turn over and it was dropping down to around 8, so obviously not enough to get the starter to spin.
Now I've changed out the battery and starter solenoid for new it's starting with no issues.
Lesson's learned =
* just because the battery is on a maintainer doesn't mean it will keep the battery in 100%, the battery is about 4 years old, and is a summer rider bike so stays on a maintainer a hell of a lot of the time.
*12v on check multimeter check doesn't mean the battery is good, see what it does when you press the button. And follow it to the problem ie: start at the battery 👉, follow it to the next point 👉 starter solenoid, 👉 the solenoid has X2 in's and one out - power in from the battery - the starter button power in to connect the terminals then power out to the starter. 👉 The next points after that would of been check the voltage/amps at the starter motor, but obviously I didn't need it this time fortunately.
In the summer I'm doing a full strip down and overhaul of everything so will check all the connections to see if everything is good, bad, in tolerance etc.. as stated, it is a 98 model, original wiring, so things will obviously be weaker in areas.
You can buy stronger Lithium batteries that will give you better start power etc.. but also remember Lithium do not like the cold or heat, -5 they will break, and +20 you could end up with a fire 👀 so be warned 👍