Fear is an emotion on the anxiety spectrum.  The types of interventions that have been shown to be the most successful for anxiety are cognitive behavioral interventions (aka CBT).  The basic premise is that emotions, thoughts and behaviors are all interconnected.  The goal for your daughter will be figure out what are the thoughts she has just before she attempts to start tumbling and learn to challenge the truth of those thoughts.  Once she proves to her brain that the thoughts are incorrect, her anxiety should resolve and she should be able to tumble again like before.  For some assisstance in breaking the association between tumbling (the behavior) and the thoughts, check out The Feeling Good Handbook, section 3.

Also, you could consider taking her to a therapist for a short course of CBT if, with your help, she is unable to identify the thoughts that underlie her anxiety.    Does she have any other symptoms of anxiety?  If so, I would definitely recommend a full evaluation to make sure nothing is being missed.  Hope this helps!!!  Please keep me posted on how she does...