G'day and welcome donq69!
To put the slide back on the frame, first make sure that the safety is in the ON position.
Then pull the front of the trigger guard down from the frame and gently push it to one side so that it does not move back upwards.
Slip the slide over the end of the barrel and pull back until the slide skips over the end of the frame and engages ( or catches onto) the frame.
This should put the gun back into working condition.
also here is a video that i think will help