I entered the phrase "safe mode" to the search engine on Particle support. I have to say I see many people who cannot go into safe mode with their chips. I am not positive of the commonality of these separate issues.
One unfortunate commonality goes as follows: The original poster lays out the problem. Then many people come in with suggestions, none of which work. Then several other people pile on saying THEY have the exact same problem. The unfortunate part is that none of the threads end in a solution. What happened to all these people? Did they simply give up? Did Particle fix the problems, somehow, but nobody published what the solution was?
This snip below resonates with me. The guy had a working system and, kaboom!, next day he cannot go into safe mode. I didn't add up the people who have this problem (either the OP or folks that piled in) but it looks to be a dozen or so.
--jim