I've handwashed 100% silk items before with no problems at all - even ones that said dry-clean only. It would be completely impractical for me to spend a fortune on dry-cleaning a silk top after each night out so I just handwash carefully. I think it depends on the type of silk?
Maybe if the dress is very expensive you should play it safe and take it a dry-cleaners that specialises in evening wear.
If you do decide to handwash it, use cool to luke-warm water, NOT hot water and a gentle detergent.
Google handwashing silk and you'll see plenty of advice. Good luck.