Yes. The first place to start is by looking in storage/logs/laravel.log. You can also temporarily enable debug mode in your .env file. Some users find the latter does provide information and others see nothing. But if you have ssh access, please start by looking at your laravel.log and sharing what you see there.
If there is a lot to share, please use the code block in the reply editor or attentively, use three back ticks to start and end the section. It makes it easier to read.
ok - found the issue in the laravel.log with chatGPT to the rescue.
production.ERROR: fopen(/var/www/html/ninja/storage/framework/cache/data/ac/ac/acace746aba0910583cdd0d39bf52f9740425b1c): Failed to open stream: Permission denied
File and directory permissions were set correctly on initial install and for some reason permissions changed.
Since resetting all permissions I am able to login again. Will update if this changes in the future.
I do have a question regarding cron - the invoice reminders seem to be flaky - can you state the cron jobs that need to be set up ie format to ensure invoice reminders work correctly.