This is probably a case of the 5V rail shorting to ground somewhere/somehow. With the power removed from the board measure the resistance between the 5V pin on the regulator (the one currently at .8V) and ground. I’m guessing it’s probably some very low resistance. If so, scan the entire board with a magnifying glass looking for any speck of solder or short that could be keeping that rail at ground potential.
If it’s not some very low resistance then the voltage regulator may be bad. Un-solder and pull up the 5V lead off the board and measure the voltage on that pin when it’s not connected to anything. If it is still not at 5V then the regulator is most likely bad.
But also make sure the center pin of the regulator is really connected to ground.
…before everything else make sure the voltage regulator is orientated properly and it’s the correct part (not a 2N3904).
-Brach