The State Department says Clinton became dehydrated while dealing with a stomach virus and fainted.
When she passed out, she hit her head and sustained a concussion.
Clinton's doctors have suggested she spend the next week working from home.
But, the State Department says Clinton is looking forward to getting back into the office very soon.
A State Department official added that Clinton will not testify on September's deadly attack on four Americans in Benghazi, Libya this week as scheduled.
She was expected to appear before the Senate and House foreign affairs committees.