That means something’s wrong with the theme’s code. The code was written is such a way that the PHP interpreter (the program on the server that reads and carries out the code in your PHP files) couldn’t properly read the file and figure out what it’s saying.
Most of the times when I get this error, it’s due to a missing curly brace at the end of a function, loop, or class (thus, it reaches the end of the file before all these elements are closed). If you know PHP (or C-like syntax, possibly), you can open up the file and see if you can find the error that exists in that file.
What customization(s) did you make to theme, out of curiosity? Was it a code change or something in a settings menu? Does an un-customized version of the theme work ok? If so, then the error is likely due to the customization(s) that were made. Otherwise, you can try contacting the theme’s creator directly or post in that theme’s section of the forum to see if someone else is also experiencing this error.
I hope this helps!
P.S.: Even with a broken theme, you usually can log in by directly accessing the Admin Panel at (yoursitepath)/wp-login.php.
