decreased engine power light on 2001 GMC Sierra 2500 HD

i got my truck washed today and they locked the keys in it after using a slim jim we got into the truck with no problem but now the service ngine soon light and the decreased engine power light is on does anyone know how to reset the computer or how to get the truck to run right?

Asked by for the 2001 GMC Sierra 2500 HD
Best to first retrieve codes and note "what the computer sees is wrong". Disconnecting the battery is a very poor alternative to checking codes stored with a scan tool and then using a scan tool to reset the computer memory.