Hi there,
Thank You so much for your help and reply.
I contacted Bluehost Wordpress Support just as You suggested and in fact the Activello theme caused the problem, but when I reinstalled again it did the same thing. Bluehost Support now changed my theme temporarily but I really hoped that I could get Activello back on my site.I fell in love with this theme, seriously 
But I don’t know what’s wrong. I find a similar topic to mine here as well:
https://colorlibsupport.com/t/error-500-message/
But when I tried to get activello back up on my site again, the site crashed again. Now I’m scared to even trying it again.
Do You have any ideas why this happened? And with just a few Activello users?
Thanks again!
Best wishes!
Melinda