Roissy is a big airport with plenty of hotels around it. Depending on your budget and style, you can select the best one for you :
- Sheraton : connected to the airport terminal
- Hyatt Regency : has a shuttle to go to and from the airport from the hotel, it has an indoor pool
- Hilton : also has a shuttle and an indoor pool
There are plenty of other hotels as well :)
Then, you have a page on the airport website that describes where to go to find the shuttles.
I am pretty sure that you can find desks in the airport that will assist you in finding an hotel if you can't make an online booking. I believe there are phones to call them directly from the airport as well.
Last but not least, a lot of hotels are offering refundable bookings. So you can make a booking now and then just cancel it if you realize that you won't need it. But don't worry, I am 90% sure that you'll find an hotel in Roissy on last minute. And if you don't, just get outside of Roissy, there are nice hotels in Chantilly for instance.