I checked our network alerts, and I don't show any outages for the San Francisco area. To really isolate your issue, we need a few more details on your situation. Are you able to pull a 4G signal outside the building you are in or is this an indoor coverage issue? Were you able to get a signal here before and now it's all of the sudden gone?
There are a few things you can try on your phone to have it search for the right signal again. The easiest thing to do is a battery pull. Some people also have luck with forcing the phone into airplane mode in the settings menu and then disabling airplane mode. If you're still experiencing the issue, please post back with troubleshooting steps you've already tried, and more details of your issue.
Thanks for posting!