If your WAMP Server System tray Icon turned into yellow 
and not working, Try following steps.
1. Click on WAMPServer System tray icon > Restart All Services
If icon still appears yellow,
2. Click on Icon > Apache > Service > Test Port 80 and See which program blocks WAMP/Apache default port 80
In my PC, I have installed Skype and it uses port 80 to do communication. You can change it by Opening Skype > Tools > Options > Advanced > Connections
Uncheck “Use port 80 and 443 as alternatives for incoming connections.” And Restart WAMP.
If problem still exists,
3. Click on WAMP icon > Apache > httpd.conf
Search for “Listen 80″ in the file. and replace “80″ with “81″ one or some other port number you want. And restart WAMP.
If default port is changed, you will have to add port number at the end of “localhost” to access. Ex – “http://localhost:81/”
