Two things to check here first. Is the ignition on? No, really. Has the immobiliser been de-immobilised and obviously, you wouldn’t have left the kill switch on (again) would you?
After the ignition check the fuel. Sounds obvious, but on many older bikes there’s a tap to switch between on, off and reserve.
Next up, on older bikes, check the choke. Is it on? If it still won’t start, then the most likely cause is a weak battery. Many bikes (especially when fitted with an alarm), are hard to start after being left for a while because the voltage in the battery drops below that required to turn the engine over fast enough and provide a fat enough spark to fire a cold engine. Because the motor is turning, you might think everything is okay. Some bikes are more sensitive to this than others – Honda’s VFR400R is a particularly bad one. Eventually the battery goes flat or the spark plugs get wet with unburnt fuel. In the first case you might be able to bump start the bike, in the second, you’ll need to remove the plugs and dry them with a hairdryer.