Hi Brad,
There are a lot of things that can cause this so I'll try and give you a list of some easy things to check.
1. Make sure you on running minecraft version 1.12.2
2. Make sure your minecraft launcher is up to date and you are using a vanilla version of minecraft.
3. Try restarting your computer
4. Try un-installing and re-installing your minecraft launcher
5. Try Restarting your router
If none of these simple fixes work, take a look at the website linked below, it has some more complicated solutions. Let me know if any of these work!