If a cloud services provider goes bankrupt, how can customers retrieve their stored data?

It all depends on the terms of insolvency. Most companies will provide you ample of opportunity to take your data/business elsewhere. However, this is not guaranteed. You can ask your cloud provider for the legal document (usually in their TOS) for their insolvency terms.

Most companies that do shutter down, do try to provide customers with an decent enough time window to port their data out.