"unable to load the city at this time. Please try again."

The explanation I have seen most is that something happens during exit of your city and the city file gets corrupted somehow (say, you lose internet connection to the server before it's done saving).

Have you claimed another city in the region? Sometimes you can go into a neighboring city, go to the region view, and then enter the problem city from there. 

Darn it! Here are a few other things to try:

1. Make sure your firewall and antivirus are not blocking SimCity.exe, and that it is added to the exception lists. I know when mine updates, it occasionally likes to change settings for no good reason. If you have BitDefender installed, disable it.
2. If you have any mods installed, remove them
3. Set the desktop icon to run SimCity as administrator
4. In Origin, right-click SimCity icon, Game Properties, check the box that says "Disable Origin In-game"
5. In Origin, right-click SimCity icon, choose "Repair Game"
6. Run SimCityRecovery.exe Default location is \Program Files (x86)\Origin Games\SimCity\SimCityRecovery. This can take a while
7. Delete everything in Program Files (x86)\Origin Games\SimCity\SimCityUserData. The game will download 
8. Delete all files in C:\ProgramData\Origin\DownloadCache (You might have to go into settings and enable viewing system files: Control Panel - Appearance and Personalization - File Explorer Options - View - Hidden Files and Folders - Check the button "Show hidden files, folders and drives" and un-check "Hide protected operating system files")

If none of the above works, you could try calling support to see if they can do a roll back. Otherwise, you may have an unrecoverable city. I hope this is not the case. 

Took be AAAAGES to work this out. Good old Windows.

Now works for me, but suspect this is just a new version of this original issue, given the OP date.

Had to release a few blocks through OneDrive Documents sync too after this.