Phone hasnt been working.

For a while my phone has been powering on and off on its own, and whenever it turns back on it wont connect to internet. The times that it does connect to the internet, apps are still slow and wont work properly, usually lagging me out. I cant get my phone to work long enough to back it up and reset it either so im not sure what i should do. My phone dosent have a cell plan, so i wonder if that has to do with it.