If the computer was struck by lightning, there is a fair chance components are dead.
Almost guarantee the power supply is shot, if power surged any further, motherboard next, including cpu and potentially damage to the gpu. Powered off is irrelevant if plugged in, although it could likely have prevented further damage than the power supply.

First step would be diagnosing if you're receiving power thru the PSU, lights on mobo, case, fans spinning, etc. If no power, replace PSU.

Second step would be to diagnose if you're receiving output from cpu/gpu when they receive power. If you have case lights, no error codes, and the computer /seems/ to have booted, fans on, drives kicking into gear, general.. computer sounds.. booting into bios, etc. Most newer GPUs have an LED near/around the plug to indicate if power is being received, but an incorrect amount is present. Either causing issues to boot, or no boot at all.

Depending on your CPU, you can try plugging outputs into the standard IO ports of the motherboard, if your CPU does not have integrated graphics, standard port off the GPU. You may also try booting if you made it to this point without the GPU. If you can't get it to boot into bios, the next part I'd recommend replacing the the MOBO.

Upon replacing the motherboard, it'll be much clearer what further damage has occurred. It's at this point you'll be able to confirm if PSU+MOBO replacement is enough to get you booted, or if there are further components damaged.. RAM, HDDs, SSDs, NVMe, all requires power to run.. If it ran rampant in the system, these are all likely to need to be replaced.

I don't know a shop I'd recommend to look into this, because given the circumstances, it'll likely crawl back to building a new PC. Unfortunately this circumstance is about the worst thing that can happen to a PC. Best of luck, I hope you're able to get it running!