How can the initial "booting" state of a computer be different at different time points? In my experience, sometimes a computer boots normally, and in a time span of days, the computer won't boot, then again, it will boot normally again. I am not a computer science expert so I assume that if a computer boots normally one time and then it doesn't, it must be due to any changes made into the computer between those time points.

My question has the motivation of understanding the booting process of a computer at a basic level as to know how to debug such a behavior. 

For a concrete example, I got a new computer with Ubuntu 18.10 that presented this behavior, all the time without access to internet, so no updates/upgrades possible. 

Can anything change in a computer without the user modifying something, also while not having internet connection?
Can anything change in a computer without the user modifying something, also while not having internet connection?


Yes, definitely - computers update their own states all the time, and some of these changes will be persistent from one boot cycle to the next.

The computer itself is a complex piece of hardware, and the operating system is a complex piece of software. Both the computer (at a hardware level) and the operating system will carry out a series of self tests during the boot process. The results of some of those self tests will be stored in non-volatile memory (e.g. in some form of registry). When the computer is re-booted, the results of the self-tests from the previous boot attempt may affect the route taken through the boot process the next time around. This might avoid a problem encountered in the previous boot cycle (e.g. by loading a previous version of a corrupted driver), but it may also introduce a new problem not seen before.

In short, part of the boot process will be a check to see if the previous boot cycle was successful. If not, there may be logic in the boot process that tries a different sequence of actions, or boots into a restricted "safe" mode.