Thanks so much for your help, I really appreciate it.
I tried removing the GPU from my device manager and scanning for changes, and tried putting the old HDD back. No luck on either of those.
I tried running Unigine's GPU benchmarking software (
https://benchmark.unigine.com/superposition), and that crashed too.
Probably my next attempt will be to put the SSD back in, back up files to the HDD in an enclosure, then factory reset the system and hope for the best.
