The boot process of a modern operating system is certainly complex, even if the specific details are normally hidden out of sight from you. It’s also well tested and most of the time it works just as it should. Of course, “most of the time” usually applies to other people, so what can you do when things go wrong for you?
The first step is to disable any splash screen so you can see what is going on. If a particular service is failing, then you can try booting into runlevel 1, single user mode, and then disabling that service.
The upshot of this approach is that you may end up with a slightly crippled machine, but you’ll be able to work with it to fix any problems. If you have set up static network addressing, check your settings carefully, or switch to DHCP temporarily.
Don’t go changing things at random in the hope of fixing the problem, only to find out that the reason your Internet connection no longer works is that some workmen have cut through the lines in the street outside!