Try not to worry about it. All babies learn to roll over at about the same age and once they do there isn't much you can do to stop them. Trying to prevent a baby who wants to roll from doing so will only cause them to be upset. If she gets stuck, like she did last night, she will cry, like she did last night. You shouldn't find this happens too often and will soon stop as she gets stronger and learns to unstick herself. Don't worry, this is a natural phase that all babies go through and they come out of it ok.
Pretty much what Ruth said. It's a phase they have to go through to get to the next bit wherein they can roll freely and right themselves.
The danger of putting something there to stop them is that the desire to roll is developmental and they will try anyway. You absolutely don't want them getting tangled in something that shouldn't be there. Leave the cot clutter free and she'll get there.