DTU Limit dropped to 0 while Usage stayed at 100%. DB became unresponsive and app returned 500s consistently

Question

The DTU Limit dropped to 0 while Usage stayed at 100%. The DB became unresponsive, and the app consistently returned 500s. But earlier, we had behavior when the DTU Usage was 100% but the DTU limit was 100, an application was a little slower than usual, but in working state. Was this the Azure issue or my application when the DTU Limit dropped to 0?