auto log off problem

I have just upgraded my desktop PC from windows 8 to 10, it has gone OK with a couple of minor issues which I am sure I will solve in time (can't find control panel in the new style start button etc). However one issue that is really a pain is whenever I go away for a short time the windows screen is displayed and I have to login again. That's not so bad but all my open tabs in firefox have gone, anything not saved is lost unless office "autorecovers" the spreadsheet or word document enabling me to re-save it and off course I am waiting a little whilst windows loads up my account again.
With windows 8 I used to login to my account and leave it up and whenever I returned to the machine my browser tabs were there, my spreadsheets and docs were there ready to update. I leave my machine on and even the next day I could just nudge the mouse to activate the screen and everything is there ready to go. I am beginning to wish I had stayed with windows 8.
I have searched the forums trying to find a solution but haven't found one yet. anyone any ideas?

just watched my PC produce a blue screen saying there was a problem and is restarting so it appears I do not have a logging off problem, this must have happened earlier when I assumed it was logging me out when the system was idle. Instead it seems windows 10 has a problem and has been shutting down and restarting.
I looked at event viewer (and bear in mind I am not at all technical), it tells me the system has rebooted from a bugcheck, the error was in SYSTEM Kernel-power. I selected event log online help but it said there was no information on this event.
I think I need to get back to windows 8 which I have been happy with so I hope it is possible to undo this upgrade.

I have successfully managed to revert to my old version of Windows. Everything it working normally, very relieved!