Not sure your experience level, so pardon me if you are beyond some of my comments.

Do you have or have read the manual? A copy is here: https://www.ipdb.org/files/1414/Williams_1984_Laser_Cue_Manual.pdf

Pinwiki reference info is here: http://www.pinwiki.com/wiki/index.php?title=Williams_System_3_-_7

Read through those and familiarize yourself with the operations of the machine.

Which fuse was it? Knowing this will narrow down the problem area and we can help you better. Also, did you replace it with the right amperage or just replace what was there. Often they are wrong, so cross your fingers and hope you just put one in that was too small. Use the manual to verify the size.

A key thing to learn is the switch and coil test procedures. Switches can trigger random actions if they are stuck. Coil test can show you which coils actually work or not, regardless of game state.

Using the switch test, make sure all the balls are recognised in the trough (i'm not sure if it is a multi ball game), System 7 machines will not start without all the balls in place and with switches activated. Make sure it is on free play or has credits.

Figure out what the fuse is connected to and visually inspect all the coils, diodes, and transistors (on the driver board) on that circuit. You may have a burnt coil that has shorted. If you find one, disconnect it and see if the fuse blows.

If neither of those reveal anything, trace the wiring on that circuit to make sure nothing is shorted somewhere. Pay particular attention to ground braiding. Sometimes it works loose and touches stuff it shouldn't.

Post results her and hopefully we can help you more.