In 2018, NASA's beloved little Opportunity (oppy) was declared dead after a dust storm "killed" it. It may still be OK, but when NASA began recovery effort, they pinged oppy when it was still in sleep mode gaining power, Oppy may not have noticed any of the commands that it was sent, it probably entered "Uploss fault" which it was designed to do when it hadn't heard from earth in a long time. Maybe it's still out there... any chance???
Short answer is, it's most likely very dead.
Opportunity had several issues leading into the storm period, including failing memory. Once it was not getting enough light, the expected sequence of flat battery related problems are described here but the existing failures may mean they did not execute as expected.
Following the storm it was expected that the software would know it had been sometime without commands, and that the clock had stopped.
Therefore it was programmed to identify sunrise times and attempt to re-establish the clock and then listen at pre-defined post sunrise times for instructions. These times could not be known for certainty since they depended on exactly when 'sunrise' was being detected in the aftermath of the dust storm.
Various guesses at the time were made, with 835 messages sent at the expected windows to trigger a more complete restart.
Since no response was received, either every single one of those 835 messages failed to hit the window, or something within Opportunity (power, computer, radio) had failed while power was out and things cooled off. Unfortunately the actual model used does not appear to be on the internet to allow an actual miss probability to be calculated, but with 835 attempts it would appear to be low.