Can Not sign into App Store on my iMac.

I was already signed into App Store on my iMac and I tried to download a certain App. The App would not download. It said "Get" I clicked that. Then it said "Install" I clicked Install and the little circle went round and around and nothing no download no new app on my iMac. So I signed out of the App Store and now I can't even sign back into the App Store at all. I keep trying to sign in to the App Store. It asks for my Apple ID and password I put in the password and the Apple ID and password box just disappears and I am Not signed in. My Safari has also been running slower than normal on occasion. I've seen this on a couple other posts on here. One guy couldn't log into App Store on his iPhone either. On the communities here I searched "App Store" and posts came up for this problem especially the first post that comes up under the search box. Weird. It's like the App Store has crashed. What is going on???