Dell Vennue 11 Pro 7139 - no bootable device found

I started my tablet, after not using it for about 6 months.

It starts as it always does, the screen lights up, the Dell symbol shows and then I get a black screen with white text instead of starting up properly.

The following are the steps I took and the screens and messages which the system showed. Whatever I do, nothing seems to work.

No bootable device found
Press F1/VolumeUp key to retry boot.
Press F2/VolumeDown key for setup utility.
Press F5/Home key to run onboard diagnostics.

*F1 =》to no prevail

---×---

*F2

System information
Signed Firmware Update is enabled

Memory information
NOTE: Due to an amount of memory being assigned for system use, "Memory Available" is less than "Memory Installed". Note that certain operating systems may not be able to use all the available memory.

*Load Defaults =》to no prevail
*Exit & automatic reboot

---×---

Invalid configuration information - please run SETUP program.

Time-of-day not set - please run SETUP program.

To continue press F1/VolumeUp key
To change setup option press F2/VolumeDown key
To run onboard diagnostics press F5/Home key

*changing date and time won't help either

---×---

*F5

ePSA Pre-boot System Assessement

Where colour bars displayed? (y/n) Y

No problems have been found with this system so far. Do you want to run the remaining memory tests? This will take about 30 minutes or more. Do you want to continue? (Recommended)

All tests passed.

Pass PSA V4228 ePSA extended test pass

What do I do to fix this? How can I get my tablet to work again?