Its payload computer first experienced a problem back on 13 June, and the instruments were then placed in a safe configuration and science operations suspended. Since July 15, the spacecraft has been switched to backup hardware.
Nasa describes the success of the switch as follows:
The switch included bringing online the backup Power Control Unit (PCU) and the backup Command Unit/Science Data Formatter (CU/SDF) on the other side of the Science Instrument and Command & Data Handling (SI C&DH) unit. The PCU distributes power to the SI C&DH components, and the CU/SDF sends and formats commands and data. In addition, other pieces of hardware onboard Hubble were switched to their alternate interfaces to connect to this backup side of the SI C&DH.