The first step would be to figure out what is making the score motor run. The switches in the long red box show you which relays can cause the score motor to run:
Sing Along ball return relay (resized).jpg
How many of those relays are active when the game is stuck in reset?

One possibility is that the game has finished resetting, but isn't able to kick the ball to the shooter lane to start the game. The O/Ball Return relay fires when the ball drains and the Outhole switch closes. When the O relay fires, it causes the score motor to turn, and should cause the Ball Return Kicker to fire. If your O relay is active while the score motor is turning, check the two switches in the smaller red boxes. They both need to close to fire the Ball Return Kicker.

/Mark

I recently bought a working Sing Along. It needed a light shop job. Just figured a quick cleaning and I'd be good to go. After putting the playfield back in it is stuck in reset. I don't have a schematic. Currently the 3rd chute relay is pulsing. I checked coin door switches and all seem fine. Any ideas?

check the fish paper on the coin switches, if they are not in place then it uses the trip wire to complete the circuit

Got it. Took me a minute to find. I bent a switch on the motor board. Must have snagged it with the jones plugs when i dropped the playfield in.