Copy the Yacy directory and try this.
Look at YaCy\DATA\LOG files and see if any Errors when starting.
If so then please report at http://mantis.tokeek.de/my_view_page.php
Delete yacy.conf in yacy\data\settings and try to start with startYACY.bat in the copied directory.
Try a reinstall of Yacy just a thought.
Failing all that just get going again on different install dir.
I have had to do it once or twice in the past 18 months.
I now run a VPS server saves me on the power bill USD $8.00/month.