Heard this from application owner:

The dbs were not able to do any work because all temporary space was full. The resolution was to restart the server, releasing the temp space and restoring access. Users received a message indicating that the temp file was full and were not able to do anything.

This has happened once that I know of in several years – we would like to know if we can re-engineer the back up process or take some other action to prevent this occurring again.