It's likely something either reverted or updated if you could play one day and not the next. A couple other things to try:
Double-check the antivirus and firewall; I know when mine updates it will sometimes just randomly drop my exceptions. Set the SimCity desktop shortcut to Run As Administrator. Reboot your computer.
Make sure your router isn't blocking the game; reboot the modem and/or router.
Right-click SimCity icon in Origin, choose game properties and check the box that says "disable Origin in game for this game"
Run SimCityRepair.exe; you can find it in Program Files (x86)\Origin Games\SimCity\SimCityRecovery (be patient, this takes a while)
Go to Program Files (x86)\Origin Games\SimCity\SimCityUserData and move everything in the folder to a temp folder on your desktop. (you can just delete everything in the folder, but I always like a way to retrieve things if something goes wrong) Launch Simcity again, it should reinstall that folder.
If you have a Radeon gpu, try installing the non-whql-win7-64bit-radeon-software-crimson-relive-17.6.2-june13 driver. If you have an Nvidia card, 342.1 still works. These are much older graphics drivers, but there are several graphical glitches caused in the new drivers.
If Windows updated overnight, you might want to try uninstalling the updates one at a time to see if any of them was the issue. That's not a long-term fix, since you need your updates, but could narrow down where the issue is.