Recut the stems!!!
If you cut a half inch or so from each rose stem before putting in your vase filled with water and (ideally) flower food, you will prolong the life of your roses and they will look more beautiful.
Cutting the stems eliminates air pockets and allows clean water to come up the stem and refresh the cells of the flower.
Stems are easily plugged up. Things that plug them: air, bacteria, and dirt. A simple cut will eliminate all this and give them a fresh start.
Other things that prolong rose life: flower food, clean water, clean vase, low temperatures.
Remember, your roses (and all flowers) truly appreciate TLC, a "tender loving cut."