I was doing thesis work with a latex template. It was working well, giving correct references(with Harvard style) in the starting, but suddenly, it started giving an error below:
This is BibTeX, Version 0.99d (MiKTeX 22.7.26 32-bit) The top-level auxiliary file: thesis.aux I couldn't open style file Harvard.bst ---line 20 of file thesis.aux : bibstyle{Harvard : } I'm skipping whatever remains of this command I found no style file---while reading file thesis.aux (There were 2 error messages)

How do I fix it? Please help.

When the problem appears, I stop for a time and then again compile my program. Then, the error disappears but doesn’t show any reference now. I have tried using different reference styles, but the same thing is happening.