Won't start, but it just started, after not starting for 2 days.

So my minivan got parked 3 days ago. I was going to use it the following day and it would not start. No clicking, just a low humming/vibration like noise occurred every time I turned from on to ignition. Lights and radio turned on. So I left it alone. Checked it yesterday, didn't start. Checked it this morning and it started so I moved it to a closer parking spot and then turned it off, tried to turn it back on right after and it wouldn't start. I have checked twice since then and it's not starting, still a low humming and the radio and everything works.