Hi, Jitanjafora -
The most common reason for a startup to be interrupted or hang partway through is an extensions conflict. To test for the possibility of that, try booting into OS 9 with Extensions Off - restart or boot, then immediately press the Shift key, keep it held down until you get the Extensions Off message on the Welcome screen.
If that boots okay, then try booting with the Mac OS Base set of extensions active. To set that, restart or boot, immediately press the Spacebar, keep it held down until Extensions Manager opens - it should do that just before extensions would normally start to load. At that time select Mac OS Base from the Selected Set pulldown menu, and continue the startup.
If that boots okay, try setting Mac OS All as the Selected Set in Extensions Manager, and restart.
If that boots okay, you'll need to solve for what extensions of the remainder of your normal set are causing the problem. To do that, first create a new set (use the Duplicate Set button in Extensions Manager to do that), name it something appropriate, okay it, make sure it is the Selected Set. Then start testing.
The fastest way to do that is by halves - turn on half the rest of your usual ones, restart to test; if that boots okay, turn on half the remainder of your usual set, restart to test. At the point when booting fails you'll know the culprit is in the last group activated - turn half of those off, and restart to test. And so on.
To make it easier, you can create a text file of the current status of extensions and control panels by selecting Save Set As Text... in Extensions Manager's File menu. You can print that, then use it as a checklist.
That sounds complicated, but it goes very fast - even if you had as many as 500 extensions, you'd be able to isolate the culprit in no more than 10 restarts.