I just upgrade to win8.1 from win8 and I can't start wamp server (2.2 x64)
It was working fine on win8
Try checking that the Apache and MySQL services are still registered as services.
Look in Windows Services for services called - 'wampapache' and wampmysql'
Or if you have WAMPServer 64bit wampapache64
and wampmysqld64
If they dont exist do the following:-
wampmanager -> MySQL -> Service -> Install Service
wampmanager -> Apache -> Service -> Install Service
Then try to restart all services.