Lorraine I happen to know this set very well, you have 2 issues with it.
The first issue with the black screen and sound on is the earthing on the invertor its poor and needs to be re-made in order for the invertor that controls the backlight to ground properly and work.
The second problem is bad firmware unfortunately the firmware on this set cannot be upgraded by yourself only a sharp service agent can do that, the only option left open to you would be to replace the mainboard (firmware runs the mainboard and is present on a chip on it).The mainboard will be around £30 to buy again I will provide details
The invertor should be done first
This may sound a little daunting to you however if you/someone you know is practical enough I can walk you through this work?let me know I will provide all details.
If your not comfortable with that then it probably is not an option to repair as sets of this size can be got very cheaply.
Let me know if you can do this or need more info
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