If like me, you recently upgraded your wallet using the "upgrade client" option inside the wallet, you may have encountered an error and been left with a wallet that wouldn't open. My error messages were something like "the procedure entry point could not be located in the dynamic link library Qt5core.dll".
This reddit post provides an explanation of the issue and a fix:
https://www.reddit.com/r/gridcoin/comments/7q7txl/inwallet_upgrade_not_working_on_windows_wallets/
In short, it sounds like Windows folks will have to download the update manually from https://www.gridcoin.us/
I did it this morning and it went smoothly. The new version seems to start up much faster too!