Storage Spaces pool will be inaccessible as long as storage jobs are running. Since these jobs can not be accomplished due to the death of the second mirror, you need to stop them forcefully in order to regain access to your data. Please run the following Powershell command using elevated privileges and see how it goes.
Get-StorageJob | Stop-StorageJob
You will probably need to run chkdsk against your remaining mirror to fix possible file system issues that may have occurred due to sudden power loss.