You can Reopen Purchase Quotation as Keyur explained but there is no option available to Reopen Sale Quotation but there is a button New copy of Quotation which will create new Quotation in draft state.

If you are using OpenERP 7 version, and you hit the cancel button then you should click on 'New copy of Quotation' button to get back your quotation.

Check out https://accounts.openerp.com/forum/Help-1/question/8623/ for a way to stop this happening in the future.

Or you could just manually change the state using the Pgadmin III client. If you want the same exact copy of your order do this:

1. Go to sale.order DB.
2. Change the SO number of your cancelled order. e.g. from "SO140" to "SO180" (doesn't matter, just a not already used number)
3. Go to settings in the OpenERP UI
4. Go to Settings --> Technical --> sequences/series & identifiers
5. Change the next SO number to 140 for Sale Order
6. Go to your cancelled SO, now SO180
7. Choose "create new copy"
8. Your new copy will now be the original SO number
9. Go back to sequences, and change the next SO number to the first free available number
10. Delete your cancelled SO to free up the number

After cancel button there will appear one more button Set to Draft. By that way you can set state to Draft.