So you wanted to enjoy your vita and tried to do anything risky? (testkit mod, downgrade, messing with system files) and the only option available in safe mode is update the firmware? Or pressing L at boot doesn't help? No worry you can still save the PSVita, most of the bricks are actually recoverable. As long as it is software and you didn't wipe os0 (unless you have enso ex)
First of all, what I always recommend is to put a pup file in ux0:data/PSP2/UPDATE/PSP2UPDAT.pup It removes the need to connect with a PC and QCMA can be unreliable. You can then use it with option 5 update the system, and update from gamecard (yeah yeah that option works with memorycard). After you solve the problem, do that so next time it'll be easier.
Try to do the battery reset combo (which will reset the syscon. Can save some cases like it did for me.)
press PWR+PS+SELECT+START+L for 15-20 secs then try to power on the PSVita
In order to reinstall the firmware, we shall update the system. "Update to 3.60" is the same thing as reinstall 3.60 as it will install the 3.60 pup. So let's begin, download the pup of your choice. Here's a little list.
UP TO DATE FW : go download it from sony and don't forget to rename it into PSP2UPDAT.PUP
Once you have downloaded the pup, make sure you have qcma installed, and put the pup file in the update directory. (look in qcma settings where is the update foledr. Usually called PSV updates.)
And change QCMA settings on the firmware to avoid the "you already have the latest version" message
You need to change the "FW : up to date" in use this version for updates into : custom. Then in the custom PS Vita version field write the corresponding version to the target firmware
3.60 : 03.600.000
3.65 : 03.650.000
3.73 : 03.730.000
And if you understood the logic you can adapt it for future updates.
That's it on the PC side. Now connect your Vita through USB mode and select the option 5 : Update this system and chose update from PC and you should be reinstalling the firmware now.
Note : If after reinstalling or during the installation the Vita turn off by itself (not talking about the case where it run out of power, it just turn off without any reason) and doesn't want to boot after you tried to reinstall the firmware for the 5th time, then it means you Vita sustained some damage (water damage usually) so it's hopeless in that case. Try do open the vita and clean every part of it then reassemble it.
If you have troubles making QCMA recognize your Vita, head to the QCMA section to know how to fix the errors.
Once you have reinstalled the firmware, redo the hack and it should be good now.