What can happen when you refuse to sign a Write Up at work because you are innocent and refuse to sign?

I was accused of something at work that I did not do. My supervisor wrote me up and I refused to sign it because I am innocent. Their is only one person who knows exactly what happened because he is the one that did it. I don't know what to do. It's his word against mine. The thing that bothers me the most is I am being blamed for something I did not do and I thought that he was a friend.
Something similar happened to me after 10 years of good reviews. My boss allowed me to write my 'side' on the write-up. Her boss was told she had to write me up. Ultimately, I quit, because the 'big boss' disliked me so I had no future-he made it miserable for me and the boss that once liked me received her bonuses from him. Money makes people sell their souls. The write-ups are just to show you've been warned-and yes, they'd like to fire you. They just made work miserable for me. Start looking...good luck
Contact Human Resources... They will be able to assist you!

I've refused to sign a write up before, then i wrote back a 1 page typed document, i requested be attached to the write up in my file. They cannot FORCE you to sign anything. Forcing someone to sign a document against thier will be coercion is against the law. however if it is an at will employer dont forget they can fire you anytime with or withotu cause, so dont get an attitude or give them reasons not to liek you. Also what someone else said too is right you can always go above thier head to human resources if u feel justice is not being done. the questino is, is it worth the fight?
Good for you. Don't ever sign something like that. If you know who did it, you better come clean with it. He does not sound like a friend to me, he wants you to take the rap for something he did! If it's your word against his, then you both are in the same boat, but don't sign anything! But do keep a written record of everything that is going on, get a copy of the unsigned document also, and if the employer retailiates in any way, contact a labor lawyer right away, especially if you get fired.