Bike does not start or turn over at all. Lights work...but not neutral light...neutral switch problem? Sidestand switch?

Check that your Engine kill switch is in the Run position.

Bike must be in neutral (with neutral light showing) to crank starter, if you try starting in gear the clutch switch must be activated (lever in) and the sidestand must be up.

Probably the interlocks preventing the start....typically on a Yamaha street bike...

First...is the Engine Stop switch ON (the kill switch)
Is the bike in Neutral and does it show as neutral on the dash....
if so it SHOULD crank....however sometimes a relay hangs up....so try the alternative..
you can start it in gear but only if...
a) The sidestand is up -theres a switch to detect its retracted to its resting place
b) the clutch is pulled in -thers a clutch switch on the lever that detects the lever is pulled in.

If the bike was in neutral but only starts with clutch in and sidestand up, there is an issue with a relay.

If none of the above allows it to start....your tip over switch may be bad, loose connection etc.
Let us know how you make out.

You don't mention the year and model...but here are some general ideas.

Most Yamahas have various safety interlocks designed into the wiring system....typically a neutral switch, a sidestand switch and a clutch switch, a tipover switch and a kill switch on the right handlebar.
The design basically is to prevent riding off with the sidestand down....so if the bike is in neutral, the neutral switch grounds to light your neutral lamp and allows for ignition and starter operation.
Is you place the unit in gear...if the sidestand switch senses the stand is still down, it kills the ignition and starter.
If the bike is in gear, starter will not operate, unless the clutch is pulled in and the clutch switch detects this happening.
Tip the bike over and the tipover switch kills the ignition and fuel pump if it has one.

So...is the igniter (black box/brain etc) getting supply voltage...if not check the ignition fuse, and the handlebar kill switch .
Is the neutral light working....if not...try starting with clutch in and sidestand up.

Even though the transmission is in neutral...is the neutral light workimg?
To satisfy the interlocks, even if the light works.....try starting with sidestand up, and the clutch pulled in.
If it now starts...the interlock ciccuit is not sensing the bike is in neutral. If the light doesn;t work....inspect the wiring for your neutral switch ....usually the circuit grounds through the switch to the crankcase so any wiring break will prevent it from working.

Well...normally with most bikes...neutral indication is only needed to start the bike while its on the sidestand.
Even though it is in neutral because it doesn't indicate it, you need to have the sidestand up, and the clutch lever pulled in to attempt a start. Make sure kil switch is set to Run.
If it does start....because of no neutral indication, it should kill the engine as soon as you try to set the sidestand down.
If it will start and run....to look for the neutral indication issue, you need to trace the harness going to the neutral switch (or gear indication switch depending on make and model).
Any chance of rodent damage to wiring while stored....just seems odd the neutral light worked before...and now it doesn't. My experience is with Yamaha, and they usually grounded the neutral circuit at the switch, so you need all wiring connections secure and no wire breaks to get neutral indication, and satisy the interlocks.

The normal interlock on Yamahas is that you cannot crank starter if the machine is in gear with sidestand down. If you flip the sidestand off, it will start if the clutch is pulled in.
The problem is your bike is in neutral but the safety interlock relays are not seeing it and this is keeping the neutral lamp off also. Inspect the wiring on the left side of chassis near the engine...you should see a harness with a single sky blue wire, thats for neutral switch. The circuit grounds down at the neutral switch which is mounted inside crankcase.Ensure the connector from switch is clean and secure...if neutral light still doesn't work, unhook connector and temporarily ground the upper section of the harness...should see light illuminate, it it does, the issue is in the neutral switch in the crankcase.
The switch screws onto crankcase and there is a spring loaded brass pin that is fastened to shift drum that rotates across the inner face of the switch as you shift gears...when you reach neutral...the brass pin makes contact under the terminal, grounding the circuit, causing the neutral light and your interlocks to be satisfied.
The brass pin wears over time so it may be losing contact when you are in neutral. if necessary replace the brass pin ("neutral point ") and the spring "compression spring") found in the shift cam fork section of the parts diagrams.

If no power going to kill switch check Ignition fuse, sidestand ,clutch & neutral switches.
If all OK suspect the starter cut-out relay

Ride Safe
Bike-Doc

19 is associated with the sidestand interlock

The bike must be in neutral or have the sidestand up. These interlock the starting cut-off relay.
Does your neutral light work? If it does not, that needs to be resolved, but bike should still start if the sidestand switch is made (stand is up).
If bike still does not start, then you may have problem with the diode assembly or the starting circuit cut-off relay.

This could be a number of things - the power is interlocked by the neutral switch/side stand relay - bike must be in neutral OR the sidestand must be up in order to get power to the fuel pump relay.

Things to check:
Fuses for ignition and fuel pump
sidestand up &/or bike in neutral (neutral light displayed)
kill/run switch in 'run'
sidestand relay
fuel pump relay
Tip Over Switch

First guesses are a weak battery or starter relay going bad. Next time it does it, keep pushing the start switch rapidly and see if it catches. If so, the relay is going bad.