Do you have a battery disconnect switch?? IF you leave your battery connected it will
run down in just a few days or a couple weeks. IF your battery is very low it won't run the slide even if it's plugged in. Leave it plugged in for a couple hours and try again.
If it works after a while you definitely need a battery disconnect switch so you aren't killing your battery every time you store the trailer. It won't last long that way.
All trailers should come with a disconnect switch. Most don't and you have to add it yourself. There are many threads here about them. Just search the site for battery switch and you'll find plenty of discussion.
Also read the FAQ here-- Why is my new battery dead?