This personal organizer model by Sharp has been discontinued. The model has been around for at least 8 years. Most of the DIY repair tips for it constitute taking an alcohol swab to the battery contacts to clean them. I doubt you are going to find schematics to use in conjunction with a multimeter with it. The black screen likely is indicative of a hardware failure. There is a low probablity you could get data from it transferred to a PC. But rather than distract you with what is likely to be a deadend activity, you might contact Sharp.
Still, here is data transfer info:
https://manualsdump.com/en/manuals/sharp-oz-640-oz-630-oz-650/256401/79. Possibly the data may be held in a flash component regardless of the battery state. And although the device may be available on Ebay for between $14 and $23, I find no schematics that might enable one to swap out parts to retrieve flash data, a somewhat imaginative solution probably requiring using of a soldering iron at least. Somewhere, a SHARP technician is rolling over in his grave at the plausibility of the idea. But I'm not the first to think of that:
https://www.cemetech.net/forum/viewtopic.php?t=12553&start=0
From page 99 of the manual for the device, the following support contact information is provided. For a SHARP representative: 1 800 BE SHARP. For the SHARP website:
www.sharp-usa.com. For the a fax: 630 3789987. For writing to Customer Assistance: 1300 Naperville Drive, Romeoville, Illinois 60446 , To Wizard Help. Given the age of the device and the likely age of the manual, I'd try the website's customer assistance contact information. It is possible a Sharp repair center might be of assistance although the item is discontinued. The good news is that newer models of the device are supported yet so some technician there may be helpful.
https://www.sharpusa.com/
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