There is a very readable question-and-answer document from the European Medicines Agency (EMEA):
Questions and Answers on Recommendation for the Refusal of the Marketing Authorisation for Lenalidomide Celgene Europe (PDF, 36K)
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As it explains:
The CHMP had concerns over the way the main study was carried out, which meant that the safety of Lenalidomide Celgene Europe was difficult to assess. In particular, because the study did not compare the medicine to any other treatment, it was difficult to determine if treatment with Lenalidomide Celgene Europe increased the risk of progression to acute myeloid leukaemia.
It also explains that patients in trials and those who petition for "compassionate use" may still have access to the drug:
The company informed the CHMP that it will continue to make lenalidomide available for patients included in clinical trials or compassionate use programmes. If you are in a clinical trial or compassionate use programme and need more information about your treatment, contact the doctor who is giving it to you.