Mac G3 Tower makes beep at startup, worked before

My Mac G3 Tower used to do the apple music at startup, but now it makes a beep, and won't let me turn on monitors. This only happened after I had opened the computer up to see something. I DON'T KNOW WHAT IT MEANS AND UNTIL IT'S FIXED I CAN'T USE THE THING!!! Could someone please tell me what this means and how I fix it? THANK YOU so much.

Are you sure you closed the drop-down door properly? (been there, done that - see my Mac specs). You need to press the side door really firmly against the casing, when you'll hear a clearly audible 'clunk' noise. If necessary, lightly slam the door closed.