I find it very suspicious that one would have a mac for two years and not use it for 18 months.
Bring up activity monitor/all processes and put it in a place where it's never covered by another window - then look to see what is taking up cpu cycles. I used this to diagnose a problem with a 2010 MBP.
I put the activity monitor in the dock.......that way I can locate/load it in case of a failing system. for as I understand it, things can occur that restrict that programs availability to load.
However being a new user of the Apple system I would not know what I could do with the activity monitor if I needed it. Just a learner, and there are so many things to become acquainted with coming from an old PC's knowledge.
putting activity monitor in the dock isn't much help in a stuck system - you need it on the screen, with all processes sorted by CPU use - then you can see what is using all the cpu time.
OK......I have now added one to the Dock.......but don't really understand what it shows and does. Seems to be lots of options of showing the info in the activity monitor.........suppose it can tell certain things about what is wrong when trouble shows up. But how to fix things would be a whole new thing for me.....would have to ask and ask on the forum for help. Hoping it does not happen to me, until I know much more about reinstalling software.