What is your location, and what phone/carrier do you have now?

Most any GSM phone sold in the last few years will work in Europe. If you are in the US and use AT&T or T-Mobile, your existing phone will probably work. You may need to call them to enable international service. It will be expensive (probably around $2 and $.50 text), but if you only need it for an emergency that is probably not a big deal and not worth the hassle of finding an unlocked phone and prepaid SIM.

If you do not have a GSM phone, you will need an unlocked quad band phone. You can find them on ebay pretty cheap, but you may not have time if you are leaving next week. Signing up for a new plan with a local carrier also may not work, as they will probably have a waiting period for international service to prevent fraud.