The reset switch (also called a keyswitch since it takes a key to work) is located in different places on different machines, but usually it is on the right hand side not far from where the pull handle is or would be. The purpose in the casino is so that a casino employee can come over to the machine when a player hits a bigger jackpot and pay them their winnings. The machine is made to stop operation when a bigger jackpot is won, that way the player or somebody walking by can't hit a button and throw away the winning hand or winning spin on reels. In this mode the machine will not play another turn. It is sometimes called a "jackpot lockup" of the machine.
The casino employee verifies the win, sees that the player is paid, then they insert the reset key into the keyswitch and turns it to reset the machine so it will start playing again. Inside the machine the reset keyswitch has a couple of wires connected to it, the wires go back to the computer in the machine and tell it to reset itself and resume normal play.
The most common key used for the reset switch is called a "2341" key because that is the number stamped on it. You can buy a replacement 2341 reset key for low cost.
If you can open the main door to the machine and find the 2 wires that connect to the reset keyswitch you can touch the 2 wires together, that has the same affect as inserting the reset key and turning it. That should reset the machine.
Here is a typical reset switch (sometimes called a keyswitch) but your machine may not be exactly the same: