CTO left without a word

Find a CTO who is more talkative. Communication is key, so if this was something this person could not do and still can't , dont bother with them anymore. It's really hurtful for a company to have people who dont or barely communicate so if this was/is an issue, don't be sad about having it resolved now even though that means you will need to find a new suitable CTO. Best of luck with that, i'm sure there's plenty fit for the job you are offering who are able to communicate properly.

Its one thing to have a technical genious as CTO, if they are incapable of proper communication, this is still useless person for the role. (maybe beter as an engineer or someone who is talking to machines instead of people..)

that someone would leave without a word is already a big indication things before weren't beign communicated properly, bottled up, and eventually this triggered this happening.

1. Wanna get the info why he walked away? - Tell the CTO: "You're fired! (assuming you're the CEO or co-founder)". Next time you know, he will give you reasons why he left. - By harvesting information from people close to the CTO.

2. Wanna continue with your endeavor or go on with your life (assuming you're the CEO or co-founder)? - Find another CTO