Your Amiga refuses to boot after changing the S directory or adding some software...

The computer stores the startup files in the S directory that are needed to Start your computer.

In the S directory there is an important file called STARTUP-SEQUENCE.
This VITAL file called "startup-sequence" tells the computer what files and commands are required in order for your Amiga to boot. If your Amiga won't boot after a change was made in the System:s/ directory check to see if these files or commands may be missing from the Startup-Sequence file or the startup-sequence, itself. Other files that may be in your S directory are User-Startup, (and on some older systems startup-sequence.HD) You may even have some kind of personal configuration that requires a different file. However every Amiga will require the file "Startup-Sequence", and is arguably the most important file on your Amiga.

To get your system to start again, you can copy a STARTUP-SEQUENCE from a floppy and place it in the S directory.
(rename your old Startup Sequence to something like Startup-OLD first.
Then use a text editor to check the contents of your Startup-Sequence, and edit it as needed and attempt to get your system working again.
Or just remove the bad program.