What if your fuse on your amp had burned out but you had replaced it with a new and now your amp is not turning on?

It depends on which fuse you're talking about. Amps use fuses in different ways.

Some of the fuses are there to (supposedly) protect the output transistors. Those are the large transistors that proved the necessary power to drive the speakers.

If something happens that pushes the output transistors too hard, rather than blow out the output transistor the fuse is supposed to blow. Unfortunately, that doesn't always happen, and the transistors fail, often shorting out and THAT causes the fuse to blow.

So... depending on what caused an OUTPUT fuse to blow, you may need to replace one or more of those transistors. (note! Sometimes output transistors need to be replaced in matching pairs. Understand what you're doing before you replace transistors)

Some fuses are in place to protect other parts of the circuitry or to prevent fire. For example, the input to the power transformer is often protected with a fuse. If that fuse goes it's an indication that something with the input or the transformer has failed. Input fuses can also indicate a failed rectifier.

Finding what's wrong is generally more than just replacing a fuse. You'll need to test some of the components, or take the amp to a tech who knows how.