Mac keeps logging me out and restarting every time I try to open

So usually when I open my mac, I just have to login and all my windows and everything are there. But now pretty often, I'll open my mac and I have to choose my user (there are two users but I always use the main one) and then when I log in all my windows and applications start loading up and it's a whole process. Why does it do that? Is it a battery issue? I will say that I have had the "service recommended" alert for a while now. If anyone knows, please let me know, thanks!