What to do if you leave a period stain on a chair in someones office how can you ever face them again if you're sure they'll know it was you?

This is nothing to be ashamed about because it is an accident,, yes it is embarrassing for you now, but not to worry, talk to the person who is in the office and explain what happened, now take some very cold water and soak the stain , DO NOT RUB THE SPOT, let it sit for a bit then sponge gently, a little soap and water will clean it very nicely. this has happened to many women over the years, , so any intelligent person will know exactly how you feel, so they will not blame you in any way.