99 Cadillac sts car problem?

I left the key to the on postion for 2 hours and since then it idles ruff and in gear really ruff any suggestions?

Was the engine running?
Did the battery go dead?

Did you need jump start to get it going? Does it start OK now? Is there a yellow “check engine light on”?

You may have reset the engine computer by removing its power trying to start the car with a dead battery. The engine will run a bit rough until the computer “learns” the engine again. 50-100 miles should be plenty unless there is a check engine light, then you have a problem that a mechanic will need to address.