Place your palm over all six guitar strings while the amp is turned on and the guitar connected. If the humming noise stops it was caused by poor guitar muting due to highly sensitive electric guitar pick ups. Work on your palm-muting technique while playing to eliminate the humming sound or replace your guitar's pick ups with less-sensitive ones.
Plug the guitar amp into another outlet, preferably one that you know to have worked without a humming sound. Plug a power conditioner into the socket and the amp into the power conditioner if the humming persists.
Unplug all effect pedals and equipment from the amp and plug the guitar directly into the amp. The guitar should be the only thing plugged in. If the humming stopped, plug in one piece of equipment at a time until the humming resumes. Identify the effect piece that's causing the humming and adjust the settings on it. If the humming continues, you will need to replace it.
Turn all the control knobs to zero on the guitar amp. Connect just your guitar and set the volume to zero. If the humming sound has disappeared, slowly increase the volume on your amp and guitar and begin to readjust the control settings. Adjust the setting in a manner that's different from the previous settings and that sounds good to you.
Take your amp into a guitar shop and have a professional technician examine it if all these methods failed.