I did something similar at work. Someone ( John the driver ) had been using the jet wash and put it away after using it. He switched off the water , but not the actual main power switch.
The next day I went to use and it wouldn’t fire up, so I went disconnect the hose, and a jet of steam flumed out across my hand .
Fortuantly ,having previuosly set myself on fire , I went to the sink and ran my hand under the tap for a few mins , then made a bandage out of a tea towwl and filled that with ice.
The pressure had slowly dropped in the pump head and the motor started up, but with no water the pressure didnt increase enough to switch the motor off, so it ran till the thermal cut out did its job. Unfortunalty the thermal cut out is above 100’c so any water in the system got super heated , expanded , then when I popped the garden hose fitting whooshed out all over my hand. I knew something smelt hot at work but couldnt locate the source
Hope you heal quick, burns / scalds are nasty and theeres not really alot you can do with them .