That is strange 
. Did you since then by any chance installed a new anti virus software, or run your antivirus and deleted something? I do not know of Grin++ having had any problems with virus scanners before, but it is the only thing that comes to mind.
Perhaps try deleting your Grin++ node folder, maybe something got corrupted by an accidental shutdown or something. You find the folder here:
C:\Users$YOUR_USERNAME.GrinPP\MAINNET\NODE
Try deleting that folder and running it again. In case you did not do so, also try restarting your computer.
If that does not solve it try asking on the Grin++ Telegram support channel