I have a USB type C charging cable that I charge my cellphone with. Couple of days ago I found out that it got frayed on some parts of its plastic cover. I always charge my cellphone in bed and before sleeping. I'm kinda worry right now that I might have rolled onto the frayed part while sleeping and got electrified without noticing it due to the fact that the cellphone chargers output voltage is too low to be felt ( I read this somewhere ) but being electrified for hours ( when sleeping ) even by a low voltage must be really dangerous I suppose. Something I feel heartburn and shortness of breath but I've always had these problems and don't know if they are related to the charger story or not.
Can 5V kill you?
Short answer: YES ...BUT....you'd have to broach the skin to subcutaneous layers or have lots of blood exposed in positions either side of the heart. So with a frayed USB cable (both ground and +5V exposed) an impossibly low risk for such local contact.
If your skin is intact the likely resistance is in the 10-100 kOhms range, so death would be impossible (insufficient current flow).
What are the chances of a frayed USB cable killing me on contact?
Short answer: Impossibly low risk
I didn't say there is no risk because the situation is complex.
If all that is wrong is the DC cable is frayed and you are not bleeding or have open wounds for it to touch ….the risk is infinitesimally small. Current would flow only locally in the area of contact and you'd be unlikely to even feel it.
However the cellphone charger is connected to the AC mains 120/240V AC and the isolation/grounding on some chargers is questionable. Under these conditions you could get higher exposure ....but you'd need some other contact (typically a ground) on the other side of the body to get current to flow across the heart.
For example, picking up this frayed USB cord (and making contact subcutaneously) with the charger plugged in while standing in a bath of water with the taps on (where the water may well have a path to earth) could be very problematic with a poor charger design (but this is no longer a +5V problem).
Get a new charger or replace the cable. It is not worth the risk no matter how low the risk is.
...and put your cellphone on the nightstand for goodness sake....
You might also read this Q/A.
Being exposed to 5V for extended times is absolutely harmless. Nothing to worry about.
However, you should still replace the cable. A frayed cable can create partial for full shorts and these can damage the charger or the phone and result in things overheating or breaking. The cable connector itself could get hot too.