My DVR has been working fine for the past 2.5 years.
[Potentially relevant information: About 3 weeks ago, I tried playing back some recorded files but received the message "File not found", which was odd. I've been trying to figure this issue out.]
This evening, I realised that there was a small black Logitech item inserted in the front USB port. It looks like a nano receiver for a wireless mouse — due to poor memory, I can't accurately confirm what this USB item is. I pulled it out, Amcrest system was still working.
When I walked by the system 1hr later, the monitor screen was in standby mode, apparently not receiving any signal from the DVR. The screens on the Amcrest phone app show the text "Open failed".
I've tried removing and reattaching the DVR power cord.
The green power light in front is glowing steadily. The HDD light is off.
There doesn't seem to be any odd noises coming from the DVR.
I've tried opening up the DVR and performing a hard reset but there's no response or noise when I press and hold down the button.
Any suggestions on how to troubleshoot/fix

Thanks in advance.