Sounds like it could be a power supply issue.

A quick search using the Google link at the top of the page will find many threads about 800 series power supplies.

If you are willing/capable, I would take the lid off and check the outputs of the power supply as a start.

If it is the PSU and it has completely failed it may take other components with it (capacitors from memory).

Not that big or expensive a job.

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