Hi @tsik64177
Try a power refresh and check if that resolves the problem.
Turn off the AIO if it is on and then disconnect the power from the AIO.
Open the AIO and remove the coin cell RTC battery from the motherboard.
Here's the [https://dl.dell.com/topicspdf/optiplex-9020-aio_owners-manual_en-us.pdf|owner manual] that may help. Go to p.16 to view the procedure to remove/replace the coin cell battery.
''Press and hold the AIO's Power button for 30 seconds and then release it.''
Reinsert the coin cell battery (+ve side up as marked on the battery), reassemble the AIO, reconnect the power and turn it on and check if it now works OK.
If it starts OK you ''might'' get a message stating that the date and time are incorrect. This is normal as the above procedure has reset the BIOS back to its factory default settings. Once the date and time have been corrected the message won't appear the next time the AIO is started. If you have changed any user definable BIOS settings to suit whatever system requirements you had, they will have to be changed back again also.