I like to look at the simple things first: Are all wires and connections good, free of corrosion and are tight? Are batteries charged? I am surprised there is not an in-line fuse (but maybe some trailers don't have them). Has someone followed the cable from the pos terminal on the battery all the way back to the slide to check for a fuse or resettable breaker?
Here are some other thoughts:
Maybe the solenoid is bad now?
If not, the next thing I would check is the voltage to the slide-out drive motor itself. Do you have 12 volts getting to the motor?
If you're getting power, then I might try to manually open it part way. If you are not able to do this, then something is binding.
If you can open partway, try to extend it with the motor the rest of the way.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD