Cisco 871 lost its flash filesystem

My Cisco 871 router dropped dead overnight. It now wakes up with the following message:

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System Bootstrap, Version 12.3(8r)VI2, RELEASE SOFTWARE
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C870 Series (Board ID:3-148) platform with 131072 Kbytes of main memory

flashfs[0]: unable to allocate available block.
flash: file system initialization failed
Could not open boot directory: flash:/
Unable to open boot file

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What should I do to recover the filesystem?
I do have the configuration file saved somewhere else.

Thanks.