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What is the output of say
journalctl -r -b -1?
Assuming you boot ONCE since the gray screen. If you booted twice, change the above number to 2 and so on depending on your number of reboots since.
EDIT: You can also take a look at this Troubleshooting System Stutter, Lags, Freezes, and Hangs
There is more than you need in there but hopefully a couple of tips on where and how to troubleshoot, althought it's not a freeze or hang, it's still close enough.