What makes one unable to go to sleep at a reasonable time? for the past few days i'm am unable to fall asleep before 12am. why? what can i do to help?

Insomnia: Insomnia may be caused by multiple conditions. Disrupted sleeping cycle by changing sleeping and waking up hours (shift work sleeping disorder) or hyperactive states (bipolar, adhd, drug abuse) or depression may be common reasons for that problem. Anxiety problems (generalized anxiety disorder, ptsd) can be responsible too. Breathjng disorders like COPD or sleep apnea can contribute.

Depends: Sleep is dependent on multiple factors. It is not just the night before's sleep or any daytime napping that affects it, it can be food/medications/illness/stress and other factors. Try to keep a log (found online) to see if there is pattern. Work on sleep hygiene which includes avoiding light and stimulation in the evening. Get up same time every morning. If no improvement see doctor.

Sleep hygiene: Most sleep experts recommend keeping your sleep patterns structured to maximize the chance of good regular sleep. Attempting to sleep at your usual bedtime, in a dark/quiet room, is recommended. Be up at your regular time in the morning, avoid napping during the day, & attempt again to go to sleep at your regular time. Relaxation techniques can be very helpful for sleep. See your doctor for help.